diff --git a/mutatest/filters.py b/mutatest/filters.py
index e7c5add..6cd3f15 100644
--- a/mutatest/filters.py
+++ b/mutatest/filters.py
@@ -125,7 +125,11 @@ def coverage_data(self) -> CoverageData:
         return self._coverage_data
 
     def filter(  # type: ignore
-        self, loc_idxs: Set[LocIndex], source_file: Union[str, Path], invert: bool = False
+        self,
+        loc_idxs: Set[LocIndex],
+        source_file: Union[str, Path],
+        invert: bool = False,
+        resolve_source: bool = True,
     ) -> Set[LocIndex]:
         """Filter based on coverage measured file.
 
@@ -138,11 +142,15 @@ class instance, and any valid measured file can be used in the filter.
             loc_idxs: location index set of targets
             source_file: source file that is measured by the coverage file
             invert: flag for inverted filter using NOT
+            resolve_source: flag for using resolved source_file vs. direct str, default True.
+                This exists mostly for testing purposes to access mocked entries in the
+                fake coverage files.
 
         Returns:
             Filtered set of location index set
         """
-        measured_file = str(Path(source_file).resolve())
+        measured_file = str(Path(source_file).resolve()) if resolve_source else str(source_file)
+
         covered_lines = self.coverage_data.lines(measured_file) or list()
 
         if invert:
diff --git a/mutatest/tests/conftest.py b/mutatest/tests/conftest.py
index 60d0e22..bb96280 100644
--- a/mutatest/tests/conftest.py
+++ b/mutatest/tests/conftest.py
@@ -118,11 +118,11 @@ def mock_coverage_file(tmp_path_factory):
 
     folder = tmp_path_factory.mktemp("cov")
 
-    # aligned to fixture mock_source_and_targets
+    # aligned to fixture mock_source_and_targets for file3.py used in positive filter.
     mock_contents = {
-        "/simple_isnot/isnot/__init__.py": [1],
-        "/simple_isnot/isnot/test_isnot.py": [1, 3, 4],
-        "/simple_isnot/isnot/run.py": [1, 4, 2],
+        "file1.py": [1],
+        "file2.py": [1, 3, 4],
+        "file3.py": [1, 2, 4],
     }
 
     mock_cov_file = folder / ".coverage"
@@ -147,7 +147,8 @@ def mock_source_and_targets():
 
     Covered lines include: 1, 2, 4
     """
-    source_file = Path("/simple_isnot/isnot/run.py")
+    # see mock_coverage_file fixture
+    source_file = Path("file3.py")
     targets = {
         LocIndex(ast_class="AugAssign", lineno=1, col_offset=1, op_type="o"),
         LocIndex(ast_class="AugAssign", lineno=2, col_offset=1, op_type="o"),
diff --git a/mutatest/tests/test_filters.py b/mutatest/tests/test_filters.py
index 970828e..be20da7 100644
--- a/mutatest/tests/test_filters.py
+++ b/mutatest/tests/test_filters.py
@@ -17,6 +17,7 @@
 @pytest.fixture(scope="module")
 def mock_CoverageFilter(mock_coverage_file):
     """Mock CoverageFilter on the mock_coverage_file defined in conftest."""
+    print(mock_coverage_file)
     return CoverageFilter(coverage_file=mock_coverage_file)
 
 
@@ -42,8 +43,12 @@ def test_unset_coverage_file(fn):
 )
 def test_filter(mock_CoverageFilter, mock_source_and_targets, invert, expected):
     """Coverage filter inverted and not inverted set filtering."""
+    # use resolve_source=False for the mocks of relative file names.
     results = mock_CoverageFilter.filter(
-        mock_source_and_targets.targets, mock_source_and_targets.source_file, invert=invert
+        mock_source_and_targets.targets,
+        mock_source_and_targets.source_file,
+        invert=invert,
+        resolve_source=False,
     )
     result_ln = sorted([r.lineno for r in results])
     assert result_ln == expected