Female polymorphism is considered to be maintained through negative frequency-dependent selection imposed by costly male harassment. However, few studies have questioned whether male harassment negatively affects female morph success and does so differently for female morphs, especially in the wild. In the present study, we quantified female morph condition (relative body mass and energy reserves) for a colour polymorphic damselfly under natural conditions and evaluated these measures against variation in proxies of male harassment (population density and operational sex ratio) and ambient temperature. Differences in protein content between female morphs were detected and the variation in condition could partly be explained from concomitant variation in proxies of male harassment. Specifically, the relationship between protein content and operational sex ratio differed between morphs in that the negative effect of male harassment was more pronounced in gynomorphs than in andromorphs. In addition, ambient temperature affected the body mass and protein content of female morphs differently, with andromorphs having higher condition values in favourable weather conditions, whereas, for gynomorphs, the patterns tended to be opposite. In conclusion, the results obtained in the present study suggest that male harassment negatively and differentially affects female morph success. Future studies should aim to elucidate whether the observed effects of ambient temperature contribute to the maintenance of the polymorphism. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 97, 545–554.