SUMMARYJn brighl sunshine, temperatures within the apex of the keel of flowers of Lotus corniculatus were consistently higher in dark-keeled than in light-keeled indi\ iduals. Temperature differences may increase with higher ambient temperatures and averaged 5*7 "C at 23-2 °C ambient. In overcast conditions, no significant temperature differences between dark-keeled and light-keeled flowers were observed, or light keel temperatures were slightlygreater, ln general, temperatures within keels were lower than ambient.Within sites, dark-keeled plants showed a tendency to inhabit colder microsites than did light-keeled plants. Contrary to some earlier reports, we believe that most clina! \'ariation for keel colour is temperature related.We suggest that there may be an optimum temperature for pollen germination and pollen tube growth, ln cold microsites, dark-keeled plants would be more fecund, but in warm microsites light-keeled plants would set more seed, thus maintaining the polymorphism found in most sites.