diff --git a/pymath/arithmetic.py b/pymath/arithmetic.py
index b559788..2b2cadd 100644
--- a/pymath/arithmetic.py
+++ b/pymath/arithmetic.py
@@ -10,19 +10,30 @@ def gcd(a, b):
         :returns: the gcd
 
         """
-        if a > b:
-            c = a % b
+        pos_a, _a = (a >= 0), abs(a)
+        pos_b, _b = (b >= 0), abs(b)
+
+        gcd_sgn = (-1 + 2*(pos_a or pos_b))
+
+        if _a > _b:
+            c = _a % _b
         else:
-            c = b % a
+            c = _b % _a
 
         if c == 0:
-            return min(a,b)
-        elif a == 1:
-            return b
-        elif b == 1:
-            return a
+            return gcd_sgn * min(_a,_b)
+        elif _a == 1:
+            return gcd_sgn * _b
+        elif _b == 1:
+            return gcd_sgn * _a
         else:
-            return gcd(min(a,b), c)
+            return gcd_sgn * gcd(min(_a,_b), c)
+
+if __name__ == '__main__':
+    print(gcd(3, 15))
+    print(gcd(3, 15))
+    print(gcd(-15, -3))
+    print(gcd(-3, -12))
 
 
 # -----------------------------
diff --git a/test/test_arthmetic.py b/test/test_arthmetic.py
new file mode 100644
index 0000000..f5cb2d6
--- /dev/null
+++ b/test/test_arthmetic.py
@@ -0,0 +1,42 @@
+#!/usr/bin/env python
+# encoding: utf-8
+
+
+import unittest
+
+from pymath import arithmetic
+
+
+
+class TestArithmetic(unittest.TestCase):
+    """Testing functions from pymath.arithmetic"""
+
+    def test_gcd_commu(self):
+        self.assertEqual(arithmetic.gcd(3, 15), arithmetic.gcd(15,3))
+
+    def test_gcd1(self):
+        self.assertEqual(arithmetic.gcd(3, 15), 3)
+
+    def test_gcd2(self):
+        self.assertEqual(arithmetic.gcd(14, 21), 7)
+
+    def test_gcd_prem(self):
+        self.assertEqual(arithmetic.gcd(14, 19), 1)
+
+    def test_gcd_neg(self):
+        self.assertEqual(arithmetic.gcd(3, -15), 3)
+        self.assertEqual(arithmetic.gcd(-3, -15), -3)
+
+
+if __name__ == '__main__':
+    unittest.main()
+
+
+
+
+
+# -----------------------------
+# Reglages pour 'vim'
+# vim:set autoindent expandtab tabstop=4 shiftwidth=4:
+# cursor: 16 del 
+