Longevity in four inbred strains and their hybrids has been studied at two temperatures, 29�5 and 25�C. Heterosis was found at both temperatures but was more extreme at 29� 5�C, which is a very unfavourable environment for D. melanogaster. This observation has its parallel in observations on various fitness factors in several organisms. At 29� 5�C there was more variability of a genotypic nature between the hybrids than at 25�C, perhaps because the adaptation to this unfavourable environment depencls on rather special gene combinations. Thus longevity varies between genotypes, but the pattern of variation depends in an intimate way on the environment.