The hypothesis that increased melanism provides a benefit in prolonging diel activity through more efficient absorption of solar radiation was tested in the field on a population of alpine Colias sulphur butterflies. A marked increase in the duration of flight and feeding behaviour existed for melanistic individuals when compared to lighter individuals under cool temperatures and low intensity solar radiation. More melanistic butterflies moved longer distance per day, and emigrated from the population at a faster rate. At high temperature and high radiant load lighter coloured individuals appeared more active. This is the first field demonstration of the advantage of melanism for increasing activity of ectotherms in cold environments.