diff --git a/CHANGELOG.md b/CHANGELOG.md
index 70c1c87..cad8eb9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,12 @@
 
 All notable changes to this project will be documented in this file.
 
+## v0.3.1
+
+### Changed
+
+- Use `__slots__` to optimize memory usage.
+
 ## v0.3.0
 
 ### Added
diff --git a/mp_yearmonth/yearmonth.py b/mp_yearmonth/yearmonth.py
index b24ac22..cd2c3c0 100644
--- a/mp_yearmonth/yearmonth.py
+++ b/mp_yearmonth/yearmonth.py
@@ -41,6 +41,8 @@ class YearMonth:
         month (int) : The month.
     """
 
+    __slots__ = ("year", "month")
+
     def __init__(self, year: int, month: int):
         """Create a new YearMonth object.
 
diff --git a/pyproject.toml b/pyproject.toml
index 2b236e0..1500082 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "mp-yearmonth"
-version = "0.3.0"
+version = "0.3.1"
 description = "A year-month datatype for Python."
 keywords = ["year", "month", "date", "calendar"]
 authors = ["Ramon Torres <ramon@macropoetry.com>"]