Burr medic (Medicago polymorpha L.) is commonly found in Sardinia and in Chile throughout a wide range of Mediterranean type climates. In Sardinia it is part of the native flora at altitudes ranging between 0 and 1000 m a.s.l., with annual rainfall ranging between 500 and 1100 mm. In Chile, burr medic is naturalized and distributed in a north-south gradient of approximately 1000 km, where annual rainfall ranges between 100 mm (in the north) and 1300 mm (in the south). Ecophysiological studies on populations of M. polymorpha collected in these two countries revealed some common but also different trends of adaptation to a temperature-humidity gradient. Days to first flower and total dry matter were positively correlated with precipitation of the collection site (PP) in Chile but not in Sardinia. The percentage of hard seeds was negatively correlated with PP in both regions. The ecological and agronomic implications of these results are discussed.