Variation in the degree of insect wing melanin affects thermoregulation, and is expected to be adapted to local environmental conditions, for example over an elevational gradient. The effects of melanization on flight activity and egg maturation rate were assessed in the closely related butterflies Colias philodice eriphyle and C. eurytheme using experimental manipulation of wing darkness and transplant experiments between high and low elevation sites. Experimental manipulation of wing darkness in C. p. eriphyle demonstrated that light males had reduced flight activity at high elevations, and darkened males had reduced flight activity at low elevations. In contrast, the transplant experiments revealed asymmetrical adaptation for male C. p. eriphyle. At high elevations darker, high-elevation males had higher flight activity than lighter, low-elevation males, but there was no difference between the two groups at low elevation. For females, melanization had no effect on flight activity. However, an increase in female C. eurytheme wing darkness led to a significantly higher egg maturation rate at cold ambient temperatures, which may increase female reproductive output under natural conditions. Therefore, dispersers moving down in elevation may be more successful than those moving up. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82, 79–87.