The blue mussel,Mytilus edulis, may be found with some degree of pale striping on the shell. This variation was postulated to have an adaptive significance, with the morphs differing in the proportion of incident sunlight transformed into heat. The hypothesis was tested by studies of geographic variation of the frequencies of the morphs along latitudinal and tidal zone transects, comparison of sunlit and shaded environments, and byin vivo temperature measurements. The data generally support the hypothesis that striped individuals attain lower temperatures when exposed to sunlight, are favored where mortality from heat stress is common, and are at a disadvantage where mortality from freezing predominates.