diff --git a/tests/mixed_line_ending_test.py b/tests/mixed_line_ending_test.py
new file mode 100644
index 0000000..9b49b5e
--- /dev/null
+++ b/tests/mixed_line_ending_test.py
@@ -0,0 +1,154 @@
+import pytest
+
+from pre_commit_hooks.mixed_line_ending import mixed_line_ending
+
+# Input, expected return value, expected output
+TESTS_FIX_AUTO = (
+    # only 'LF'
+    (b'foo\nbar\nbaz\n', 0, b'foo\nbar\nbaz\n'),
+    # only 'CRLF'
+    (b'foo\r\nbar\r\nbaz\r\n', 0, b'foo\r\nbar\r\nbaz\r\n'),
+    # only 'CR'
+    (b'foo\rbar\rbaz\r', 0, b'foo\rbar\rbaz\r'),
+    # mixed with majority of 'LF'
+    (b'foo\r\nbar\nbaz\n', 1, b'foo\nbar\nbaz\n'),
+    # mixed with majority of 'CRLF'
+    (b'foo\r\nbar\nbaz\r\n', 1, b'foo\r\nbar\r\nbaz\r\n'),
+    # mixed with majority of 'CR'
+    (b'foo\rbar\nbaz\r', 1, b'foo\rbar\rbaz\r'),
+    # mixed with as much 'LF' as 'CRLF'
+    (b'foo\r\nbar\nbaz', 1, b'foo\r\nbar\nbaz'),
+    # mixed with as much 'LF' as 'CR'
+    (b'foo\rbar\nbaz', 1, b'foo\rbar\nbaz'),
+    # mixed with as much 'CRLF' as 'CR'
+    (b'foo\r\nbar\nbaz', 1, b'foo\r\nbar\nbaz'),
+    # mixed with as much 'CRLF' as 'LF' as 'CR'
+    (b'foo\r\nbar\nbaz\r', 1, b'foo\r\nbar\nbaz\r'),
+)
+
+
+@pytest.mark.parametrize(
+    ('input_s', 'expected_retval', 'output'),
+    TESTS_FIX_AUTO,
+)
+def test_mixed_line_ending_fix_auto(input_s, expected_retval, output, tmpdir):
+    path = tmpdir.join('file.txt')
+    path.write(input_s)
+    ret = mixed_line_ending(('--fix=auto', path.strpath))
+
+    assert ret == expected_retval
+    assert path.read_binary() == output
+
+
+# Input, expected return value, expected output
+TESTS_NO_FIX = (
+    # only 'LF'
+    (b'foo\nbar\nbaz\n', 0, b'foo\nbar\nbaz\n'),
+    # only 'CRLF'
+    (b'foo\r\nbar\r\nbaz\r\n', 0, b'foo\r\nbar\r\nbaz\r\n'),
+    # only 'CR'
+    (b'foo\rbar\rbaz\r', 0, b'foo\rbar\rbaz\r'),
+    # mixed with majority of 'LF'
+    (b'foo\r\nbar\nbaz\n', 1, b'foo\r\nbar\nbaz\n'),
+    # mixed with majority of 'CRLF'
+    (b'foo\r\nbar\nbaz\r\n', 1, b'foo\r\nbar\nbaz\r\n'),
+    # mixed with majority of 'CR'
+    (b'foo\rbar\nbaz\r', 1, b'foo\rbar\nbaz\r'),
+    # mixed with as much 'LF' as 'CR'
+    (b'foo\rbar\nbaz', 0, b'foo\rbar\nbaz'),
+    # mixed with as much 'CRLF' as 'CR'
+    (b'foo\r\nbar\nbaz', 0, b'foo\r\nbar\nbaz'),
+    # mixed with as much 'CRLF' as 'LF' as 'CR'
+    (b'foo\r\nbar\nbaz\r', 0, b'foo\r\nbar\nbaz\r'),
+)
+
+
+@pytest.mark.parametrize(
+    ('input_s', 'expected_retval', 'output'),
+    TESTS_NO_FIX,
+)
+def test_detect_mixed_line_ending(input_s, expected_retval, output, tmpdir):
+    path = tmpdir.join('file.txt')
+    path.write(input_s)
+    ret = mixed_line_ending(('--fix=no', path.strpath))
+
+    assert ret == expected_retval
+    assert path.read_binary() == output
+
+
+# Input, expected return value, expected output
+TESTS_FIX_FORCE_LF = (
+    # only 'LF'
+    (b'foo\nbar\nbaz\n', 1, b'foo\nbar\nbaz\n'),
+    # only 'CRLF'
+    (b'foo\r\nbar\r\nbaz\r\n', 1, b'foo\nbar\nbaz\n'),
+    # only 'CR'
+    (b'foo\rbar\rbaz\r', 1, b'foo\nbar\nbaz\n'),
+    # mixed with majority of 'LF'
+    (b'foo\r\nbar\nbaz\n', 1, b'foo\nbar\nbaz\n'),
+    # mixed with majority of 'CRLF'
+    (b'foo\r\nbar\nbaz\r\n', 1, b'foo\nbar\nbaz\n'),
+    # mixed with majority of 'CR'
+    (b'foo\rbar\nbaz\r', 1, b'foo\nbar\nbaz\n'),
+    # mixed with as much 'LF' as 'CR'
+    (b'foo\rbar\nbaz', 1, b'foo\nbar\nbaz'),
+    # mixed with as much 'CRLF' as 'CR'
+    (b'foo\r\nbar\nbaz', 1, b'foo\nbar\nbaz'),
+    # mixed with as much 'CRLF' as 'LF' as 'CR'
+    (b'foo\r\nbar\nbaz\r', 1, b'foo\nbar\nbaz\n'),
+)
+
+
+@pytest.mark.parametrize(
+    ('input_s', 'expected_retval', 'output'),
+    TESTS_FIX_FORCE_LF,
+)
+def test_mixed_line_ending_fix_force_lf(
+    input_s, expected_retval, output,
+    tmpdir,
+):
+    path = tmpdir.join('file.txt')
+    path.write(input_s)
+    ret = mixed_line_ending(('--fix=lf', path.strpath))
+
+    assert ret == expected_retval
+    assert path.read_binary() == output
+
+
+# Input, expected return value, expected output
+TESTS_FIX_FORCE_CRLF = (
+    # only 'LF'
+    (b'foo\nbar\nbaz\n', 1, b'foo\r\nbar\r\nbaz\r\n'),
+    # only 'CRLF'
+    (b'foo\r\nbar\r\nbaz\r\n', 1, b'foo\r\nbar\r\nbaz\r\n'),
+    # only 'CR'
+    (b'foo\rbar\rbaz\r', 1, b'foo\r\nbar\r\nbaz\r\n'),
+    # mixed with majority of 'LF'
+    (b'foo\r\nbar\nbaz\n', 1, b'foo\r\nbar\r\nbaz\r\n'),
+    # mixed with majority of 'CRLF'
+    (b'foo\r\nbar\nbaz\r\n', 1, b'foo\r\nbar\r\nbaz\r\n'),
+    # mixed with majority of 'CR'
+    (b'foo\rbar\nbaz\r', 1, b'foo\r\nbar\r\nbaz\r\n'),
+    # mixed with as much 'LF' as 'CR'
+    (b'foo\rbar\nbaz', 1, b'foo\r\nbar\r\nbaz'),
+    # mixed with as much 'CRLF' as 'CR'
+    (b'foo\r\nbar\nbaz', 1, b'foo\r\nbar\r\nbaz'),
+    # mixed with as much 'CRLF' as 'LF' as 'CR'
+    (b'foo\r\nbar\nbaz\r', 1, b'foo\r\nbar\r\nbaz\r\n'),
+)
+
+
+@pytest.mark.parametrize(
+    ('input_s', 'expected_retval', 'output'),
+    TESTS_FIX_FORCE_CRLF,
+)
+def test_mixed_line_ending_fix_force_crlf(
+    input_s, expected_retval, output,
+    tmpdir,
+):
+    path = tmpdir.join('file.txt')
+    path.write(input_s)
+    ret = mixed_line_ending(('--fix=crlf', path.strpath))
+
+    assert ret == expected_retval
+    assert path.read_binary() == output