diff --git a/mypyc/irbuild/mapper.py b/mypyc/irbuild/mapper.py
index 4a01255e2d5d..815688d90fb6 100644
--- a/mypyc/irbuild/mapper.py
+++ b/mypyc/irbuild/mapper.py
@@ -73,6 +73,10 @@ def type_to_rtype(self, typ: Type | None) -> RType:
 
         typ = get_proper_type(typ)
         if isinstance(typ, Instance):
+            if typ.type.is_newtype:
+                # Unwrap NewType to its base type for rprimitive mapping
+                assert len(typ.type.bases) == 1, typ.type.bases
+                return self.type_to_rtype(typ.type.bases[0])
             if typ.type.fullname == "builtins.int":
                 return int_rprimitive
             elif typ.type.fullname == "builtins.float":
diff --git a/mypyc/test-data/fixtures/ir.py b/mypyc/test-data/fixtures/ir.py
index 3776a3dcc79a..76afc1ea58cc 100644
--- a/mypyc/test-data/fixtures/ir.py
+++ b/mypyc/test-data/fixtures/ir.py
@@ -122,6 +122,7 @@ def rpartition(self, sep: str, /) -> Tuple[str, str, str]: ...
     def removeprefix(self, prefix: str, /) -> str: ...
     def removesuffix(self, suffix: str, /) -> str: ...
     def islower(self) -> bool: ...
+    def count(self, substr: str, start: Optional[int] = None, end: Optional[int] = None) -> int: pass
 
 class float:
     def __init__(self, x: object) -> None: pass
diff --git a/mypyc/test-data/irbuild-str.test b/mypyc/test-data/irbuild-str.test
index 4a4992d41a5d..3e69325a454b 100644
--- a/mypyc/test-data/irbuild-str.test
+++ b/mypyc/test-data/irbuild-str.test
@@ -1,13 +1,15 @@
 [case testStrSplit]
-from typing import Optional, List
+from typing import NewType, Optional, List, Union
+NewStr = NewType("NewStr", str)
 
-def do_split(s: str, sep: Optional[str] = None, max_split: Optional[int] = None) -> List[str]:
+def do_split(s: Union[str, NewStr], sep: Optional[str] = None, max_split: Optional[int] = None) -> List[str]:
     if sep is not None:
         if max_split is not None:
             return s.split(sep, max_split)
         else:
             return s.split(sep)
     return s.split()
+[typing fixtures/typing-full.pyi]
 [out]
 def do_split(s, sep, max_split):
     s :: str
@@ -56,12 +58,15 @@ L9:
 
 
 [case testStrEquality]
+from typing import NewType, Union
+NewStr = NewType("NewStr", str)
 def eq(x: str, y: str) -> bool:
     return x == y
 
-def neq(x: str, y: str) -> bool:
+def neq(x: str, y: Union[str, NewStr]) -> bool:
     return x != y
 
+[typing fixtures/typing-full.pyi]
 [out]
 def eq(x, y):
     x, y :: str
@@ -79,13 +84,14 @@ L0:
     return r1
 
 [case testStrReplace]
-from typing import Optional
-
-def do_replace(s: str, old_substr: str, new_substr: str, max_count: Optional[int] = None) -> str:
+from typing import NewType, Optional, Union
+NewStr = NewType("NewStr", str)
+def do_replace(s: Union[str, NewStr], old_substr: str, new_substr: str, max_count: Optional[int] = None) -> str:
     if max_count is not None:
         return s.replace(old_substr, new_substr, max_count)
     else:
         return s.replace(old_substr, new_substr)
+[typing fixtures/typing-full.pyi]
 [out]
 def do_replace(s, old_substr, new_substr, max_count):
     s, old_substr, new_substr :: str
@@ -114,17 +120,19 @@ L5:
     unreachable
 
 [case testStrStartswithEndswithTuple]
-from typing import Tuple
+from typing import NewType, Tuple, Union
+NewStr = NewType("NewStr", str)
 
-def do_startswith(s1: str, s2: Tuple[str, ...]) -> bool:
+def do_startswith(s1: Union[str, NewStr], s2: Tuple[str, ...]) -> bool:
     return s1.startswith(s2)
 
-def do_endswith(s1: str, s2: Tuple[str, ...]) -> bool:
+def do_endswith(s1: Union[str, NewStr], s2: Tuple[str, ...]) -> bool:
     return s1.endswith(s2)
 
-def do_tuple_literal_args(s1: str) -> None:
+def do_tuple_literal_args(s1: Union[str, NewStr]) -> None:
     x = s1.startswith(("a", "b"))
     y = s1.endswith(("a", "b"))
+[typing fixtures/typing-full.pyi]
 [out]
 def do_startswith(s1, s2):
     s1 :: str
@@ -165,11 +173,14 @@ L0:
     return 1
 
 [case testStrToBool]
-def is_true(x: str) -> bool:
+from typing import NewType, Union
+NewStr = NewType("NewStr", str)
+def is_true(x: Union[str, NewStr]) -> bool:
     if x:
         return True
     else:
         return False
+[typing fixtures/typing-full.pyi]
 [out]
 def is_true(x):
     x :: str
@@ -185,11 +196,14 @@ L3:
     unreachable
 
 [case testStringFormatMethod]
-def f(s: str, num: int) -> None:
+from typing import NewType, Union
+NewStr = NewType("NewStr", str)
+def f(s: Union[str, NewStr], num: int) -> None:
     s1 = "Hi! I'm {}, and I'm {} years old.".format(s, num)
     s2 = ''.format()
     s3 = 'abc'.format()
     s4 = '}}{}{{{}}}{{{}'.format(num, num, num)
+[typing fixtures/typing-full.pyi]
 [out]
 def f(s, num):
     s :: str
@@ -217,11 +231,14 @@ L0:
     return 1
 
 [case testFStrings_64bit]
-def f(var: str, num: int) -> None:
+from typing import NewType, Union
+NewStr = NewType("NewStr", str)
+def f(var: Union[str, NewStr], num: int) -> None:
     s1 = f"Hi! I'm {var}. I am {num} years old."
     s2 = f'Hello {var:>{num}}'
     s3 = f''
     s4 = f'abc'
+[typing fixtures/typing-full.pyi]
 [out]
 def f(var, num):
     var :: str
@@ -267,7 +284,9 @@ L0:
     return 1
 
 [case testStringFormattingCStyle]
-def f(var: str, num: int) -> None:
+from typing import NewType, Union
+NewStr = NewType("NewStr", str)
+def f(var: Union[str, NewStr], num: int) -> None:
     s1 = "Hi! I'm %s." % var
     s2 = "I am %d years old." % num
     s3 = "Hi! I'm %s. I am %d years old." % (var, num)
@@ -322,7 +341,9 @@ L0:
     return 1
 
 [case testEncode_64bit]
-def f(s: str) -> None:
+from typing import NewType, Union
+NewStr = NewType("NewStr", str)
+def f(s: Union[str, NewStr]) -> None:
     s.encode()
     s.encode('utf-8')
     s.encode('utf8', 'strict')
@@ -340,6 +361,7 @@ def f(s: str) -> None:
     s.encode(encoding=encoding, errors=errors)
     s.encode('latin2')
 
+[typing fixtures/typing-full.pyi]
 [out]
 def f(s):
     s :: str
@@ -410,7 +432,9 @@ L0:
     return 1
 
 [case testOrd]
-def str_ord(x: str) -> int:
+from typing import NewType, Union
+NewStr = NewType("NewStr", str)
+def str_ord(x: Union[str, NewStr]) -> int:
     return ord(x)
 def str_ord_literal() -> int:
     return ord("a")
@@ -420,6 +444,7 @@ def bytes_ord_literal() -> int:
     return ord(b"a")
 def any_ord(x) -> int:
     return ord(x)
+[typing fixtures/typing-full.pyi]
 [out]
 def str_ord(x):
     x :: str
@@ -459,13 +484,16 @@ L0:
     return r6
 
 [case testStrip]
-def do_strip(s: str) -> None:
+from typing import NewType, Union
+NewStr = NewType("NewStr", str)
+def do_strip(s: Union[str, NewStr]) -> None:
     s.lstrip("x")
     s.strip("y")
     s.rstrip("z")
     s.lstrip()
     s.strip()
     s.rstrip()
+[typing fixtures/typing-full.pyi]
 [out]
 def do_strip(s):
     s, r0, r1, r2, r3, r4, r5, r6, r7, r8 :: str
@@ -481,60 +509,99 @@ L0:
     r8 = CPyStr_RStrip(s, 0)
     return 1
 
-[case testCountAll]
+[case testCountAll_64bit]
+from typing import NewType, Union
+NewStr = NewType("NewStr", str)
 def do_count(s: str) -> int:
-    return s.count("x")  # type: ignore [attr-defined]
+    return s.count("x")
+[typing fixtures/typing-full.pyi]
 [out]
 def do_count(s):
     s, r0 :: str
     r1 :: native_int
-    r2 :: bit
-    r3 :: object
-    r4 :: int
+    r2, r3, r4 :: bit
+    r5, r6, r7 :: int
 L0:
     r0 = 'x'
     r1 = CPyStr_Count(s, r0, 0)
     r2 = r1 >= 0 :: signed
-    r3 = box(native_int, r1)
-    r4 = unbox(int, r3)
-    return r4
+    r3 = r1 <= 4611686018427387903 :: signed
+    if r3 goto L1 else goto L2 :: bool
+L1:
+    r4 = r1 >= -4611686018427387904 :: signed
+    if r4 goto L3 else goto L2 :: bool
+L2:
+    r5 = CPyTagged_FromInt64(r1)
+    r6 = r5
+    goto L4
+L3:
+    r7 = r1 << 1
+    r6 = r7
+L4:
+    return r6
 
-[case testCountStart]
+[case testCountStart_64bit]
+from typing import NewType, Union
+NewStr = NewType("NewStr", str)
 def do_count(s: str, start: int) -> int:
-    return s.count("x", start)  # type: ignore [attr-defined]
+    return s.count("x", start)
+[typing fixtures/typing-full.pyi]
 [out]
 def do_count(s, start):
     s :: str
     start :: int
     r0 :: str
     r1 :: native_int
-    r2 :: bit
-    r3 :: object
-    r4 :: int
+    r2, r3, r4 :: bit
+    r5, r6, r7 :: int
 L0:
     r0 = 'x'
     r1 = CPyStr_Count(s, r0, start)
     r2 = r1 >= 0 :: signed
-    r3 = box(native_int, r1)
-    r4 = unbox(int, r3)
-    return r4
+    r3 = r1 <= 4611686018427387903 :: signed
+    if r3 goto L1 else goto L2 :: bool
+L1:
+    r4 = r1 >= -4611686018427387904 :: signed
+    if r4 goto L3 else goto L2 :: bool
+L2:
+    r5 = CPyTagged_FromInt64(r1)
+    r6 = r5
+    goto L4
+L3:
+    r7 = r1 << 1
+    r6 = r7
+L4:
+    return r6
 
-[case testCountStartEnd]
+[case testCountStartEnd_64bit]
+from typing import NewType, Union
+NewStr = NewType("NewStr", str)
 def do_count(s: str, start: int, end: int) -> int:
-    return s.count("x", start, end)  # type: ignore [attr-defined]
+    return s.count("x", start, end)
+[typing fixtures/typing-full.pyi]
 [out]
 def do_count(s, start, end):
     s :: str
     start, end :: int
     r0 :: str
     r1 :: native_int
-    r2 :: bit
-    r3 :: object
-    r4 :: int
+    r2, r3, r4 :: bit
+    r5, r6, r7 :: int
 L0:
     r0 = 'x'
     r1 = CPyStr_CountFull(s, r0, start, end)
     r2 = r1 >= 0 :: signed
-    r3 = box(native_int, r1)
-    r4 = unbox(int, r3)
-    return r4
+    r3 = r1 <= 4611686018427387903 :: signed
+    if r3 goto L1 else goto L2 :: bool
+L1:
+    r4 = r1 >= -4611686018427387904 :: signed
+    if r4 goto L3 else goto L2 :: bool
+L2:
+    r5 = CPyTagged_FromInt64(r1)
+    r6 = r5
+    goto L4
+L3:
+    r7 = r1 << 1
+    r6 = r7
+L4:
+    return r6