diff --git a/requirements.txt b/requirements.txt
index e5acdd0..d973436 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,5 @@
 bandit
+hypothesis
 pycodestyle
 pylint
 pytest
diff --git a/tests/test_hypothesis_properties.py b/tests/test_hypothesis_properties.py
new file mode 100644
index 0000000..8b6c6b6
--- /dev/null
+++ b/tests/test_hypothesis_properties.py
@@ -0,0 +1,117 @@
+# -*- coding: utf-8 -*-
+# SPDX-License-Identifier: MIT
+# The MIT License (MIT)
+#
+# SPDX-FileCopyrightText: 2014-2026 Tim Case <bitmath@lnx.cx>
+#
+# Permission is hereby granted, free of charge, to any person
+# obtaining a copy of this software and associated documentation files
+# (the "Software"), to deal in the Software without restriction,
+# including without limitation the rights to use, copy, modify, merge,
+# publish, distribute, sublicense, and/or sell copies of the Software,
+# and to permit persons to whom the Software is furnished to do so,
+# subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+"""
+Hypothesis-driven property tests for the bitmath parser and unit
+conversions. These are fuzz-style tests: hypothesis generates inputs
+across the value/unit space and shrinks any counterexample it finds.
+
+Three invariants are checked:
+
+1. Roundtrip: an instance formatted as "value unit_singular" and then
+   re-parsed produces a Bitmath with the same underlying bit count.
+2. Lossless conversion: x.to_DST().to_SRC() preserves x.bits across
+   every (SRC, DST) pair in ALL_UNIT_TYPES.
+3. Parser exception contract: parse_string raises only ValueError on
+   bad input. A leaked IndexError, AttributeError, KeyError, or
+   TypeError would be a bug.
+"""
+
+import math
+
+import pytest
+
+# Downstream packagers (Fedora/EPEL RPM builds) run %check without
+# installing test-only dependencies. Skip the whole module when
+# hypothesis isn't on the path rather than failing collection.
+pytest.importorskip("hypothesis")
+
+from hypothesis import given, settings, strategies as st  # noqa: E402
+
+import bitmath  # noqa: E402
+
+
+# Bracketed to avoid values that Python's str() renders in scientific
+# notation. The parser splits a value/unit string on the first
+# alphabetic character, which mis-handles the 'e' in '1e+16' or
+# '1e-05'. Scientific-notation file sizes aren't a realistic user
+# input, so the test space stays in the regular-notation range
+# (approximately [1e-4, 1e16)), plus exactly zero.
+nonneg_finite = st.one_of(
+    st.just(0.0),
+    st.floats(
+        min_value=1e-4,
+        max_value=1e15,
+        allow_nan=False,
+        allow_infinity=False,
+    ),
+)
+
+unit_names = list(bitmath.ALL_UNIT_TYPES)
+
+
+@given(value=nonneg_finite, unit=st.sampled_from(unit_names))
+@settings(max_examples=200)
+def test_parse_string_roundtrip(value, unit):
+    cls = getattr(bitmath, unit)
+    original = cls(value)
+    formatted = original.format("{value} {unit_singular}")
+    parsed = bitmath.parse_string(formatted)
+    assert math.isclose(parsed.bits, original.bits, rel_tol=1e-9, abs_tol=1e-9)
+
+
+@given(
+    value=nonneg_finite,
+    src=st.sampled_from(unit_names),
+    dst=st.sampled_from(unit_names),
+)
+@settings(max_examples=300)
+def test_unit_conversion_lossless(value, src, dst):
+    src_cls = getattr(bitmath, src)
+    original = src_cls(value)
+    via = getattr(original, f"to_{dst}")()
+    back = getattr(via, f"to_{src}")()
+    assert math.isclose(back.bits, original.bits, rel_tol=1e-9, abs_tol=1e-9)
+
+
+@given(garbage=st.text())
+@settings(max_examples=500)
+def test_parse_string_strict_only_raises_value_error(garbage):
+    try:
+        result = bitmath.parse_string(garbage)
+    except ValueError:
+        return
+    assert isinstance(result, bitmath.Bitmath)
+
+
+@given(garbage=st.text())
+@settings(max_examples=500)
+def test_parse_string_unsafe_only_raises_value_error(garbage):
+    try:
+        result = bitmath.parse_string(garbage, strict=False)
+    except ValueError:
+        return
+    assert isinstance(result, bitmath.Bitmath)