diff --git a/mutmut/node_mutation.py b/mutmut/node_mutation.py
index 84a69c0d..a9218c87 100644
--- a/mutmut/node_mutation.py
+++ b/mutmut/node_mutation.py
@@ -96,6 +96,35 @@ def operator_arg_removal(
             yield node.with_changes(args=[*node.args[:i], *node.args[i + 1 :]])
 
 
+supported_str_methods_swap = [
+        ("lower", "upper"),
+        ("upper", "lower"),
+        ("lstrip", "rstrip"),
+        ("rstrip", "lstrip"),
+        ("find", "rfind"),
+        ("rfind", "find"),
+        ("ljust", "rjust"),
+        ("rjust", "ljust"),
+        ("index", "rindex"),
+        ("rindex", "index"),
+        ("split", "rsplit"),
+        ("rsplit", "split"),
+        ("removeprefix", "removesuffix"),
+        ("removesuffix", "removeprefix"),
+        ("partition", "rpartition"),
+        ("rpartition", "partition")
+    ]
+
+def operator_string_methods_swap(
+    node: cst.Call
+) -> Iterable[cst.Call]:
+    """try to swap string method to opposite e.g. a.lower() -> a.upper()"""
+
+    for old_call, new_call in supported_str_methods_swap:
+        if m.matches(node.func, m.Attribute(value=m.DoNotCare(),  attr=m.Name(value=old_call))):
+            yield node.with_changes(func=cst.Attribute(value=node.func.value, attr=cst.Name(value=new_call)))
+
+
 def operator_remove_unary_ops(
     node: cst.UnaryOperation
 ) -> Iterable[cst.BaseExpression]:
@@ -208,6 +237,7 @@ def operator_match(node: cst.Match) -> Iterable[cst.CSTNode]:
     (cst.UnaryOperation, operator_remove_unary_ops),
     (cst.Call, operator_dict_arguments),
     (cst.Call, operator_arg_removal),
+    (cst.Call, operator_string_methods_swap),
     (cst.Lambda, operator_lambda),
     (cst.CSTNode, operator_keywords),
     (cst.CSTNode, operator_swap_op),
diff --git a/tests/test_mutation.py b/tests/test_mutation.py
index 5644137c..fe3e1bb1 100644
--- a/tests/test_mutation.py
+++ b/tests/test_mutation.py
@@ -40,6 +40,32 @@ def mutated_module(source: str) -> str:
         # ('break', 'continue'),  # probably a bad idea. Can introduce infinite loops.
         ('break', 'return'),
         ('continue', 'break'),
+        ('a.lower()', 'a.upper()'),
+        ('a.upper()', 'a.lower()'),
+        ('a.lstrip("!")', ['a.rstrip("!")', 'a.lstrip("XX!XX")', 'a.lstrip(None)']),
+        ('a.rstrip("!")', ['a.lstrip("!")', 'a.rstrip("XX!XX")', 'a.rstrip(None)']),
+        ('a.find("!")', ['a.rfind("!")', 'a.find("XX!XX")', 'a.find(None)']),
+        ('a.rfind("!")', ['a.find("!")', 'a.rfind("XX!XX")', 'a.rfind(None)']),
+        ('a.ljust(10, "+")', [
+            'a.ljust("+")', 'a.ljust(10, "XX+XX")',
+            'a.ljust(10, )', 'a.ljust(10, None)',
+            'a.ljust(11, "+")', 'a.ljust(None, "+")',
+            'a.rjust(10, "+")'
+        ]),
+        ('a.rjust(10, "+")', [
+            'a.ljust(10, "+")', 'a.rjust("+")',
+            'a.rjust(10, "XX+XX")', 'a.rjust(10, )',
+            'a.rjust(10, None)', 'a.rjust(11, "+")',
+            'a.rjust(None, "+")'
+        ]),
+        ('a.index("+")', ['a.rindex("+")', 'a.index("XX+XX")', 'a.index(None)']),
+        ('a.rindex("+")', ['a.index("+")', 'a.rindex("XX+XX")', 'a.rindex(None)']),
+        ('a.split()', 'a.rsplit()'),
+        ('a.rsplit()', 'a.split()'),
+        ('a.removeprefix("+")', ['a.removesuffix("+")', 'a.removeprefix("XX+XX")', 'a.removeprefix(None)']),
+        ('a.removesuffix("+")', ['a.removeprefix("+")', 'a.removesuffix("XX+XX")', 'a.removesuffix(None)']),
+        ('a.partition("++")', ['a.rpartition("++")', 'a.partition("XX++XX")', 'a.partition(None)']),
+        ('a.rpartition("++")', ['a.partition("++")', 'a.rpartition("XX++XX")', 'a.rpartition(None)']),
         ('a(b)', 'a(None)'),
         ("dict(a=None)", ["dict(aXX=None)"]),
         ("dict(a=b)", ["dict(aXX=b)", 'dict(a=None)']),
@@ -235,15 +261,15 @@ def xǁFooǁmember__mutmut_1(self):
 
 
 def test_function_with_annotation():
-    source = "def capitalize(s : str):\n    return s[0].upper() + s[1:] if s else s\n".strip()
+    source = "def capitalize(s : str):\n    return s[0].title() + s[1:] if s else s\n".strip()
 
     mutated_code = mutated_module(source)
-    print(mutated_code)
+    print(mutated_code, open("aboba", "a"))
 
     expected_defs = [
-        'def x_capitalize__mutmut_1(s : str):\n    return s[1].upper() + s[1:] if s else s',
-        'def x_capitalize__mutmut_2(s : str):\n    return s[0].upper() - s[1:] if s else s',
-        'def x_capitalize__mutmut_3(s : str):\n    return s[0].upper() + s[2:] if s else s',
+        'def x_capitalize__mutmut_1(s : str):\n    return s[1].title() + s[1:] if s else s',
+        'def x_capitalize__mutmut_2(s : str):\n    return s[0].title() - s[1:] if s else s',
+        'def x_capitalize__mutmut_3(s : str):\n    return s[0].title() + s[2:] if s else s',
     ]
 
     for expected in expected_defs: