A pair of black-spotted sailf in mollies (Poecilia latipinna), collected from an otherwise normally pigmented population at Key Largo, Florida, was set up as a mated pair in the laboratory in order to investigate the genetic basis of the melanistic phenotype. Results of that cross and of crosses between different F1 progeny indicated that the black spotting was due to a single allele. The mutant allele showed incomplete dominance, variable penetrance in the heterozygous state, and extremely variable expressivity. Most of the variation in expressivity was determined to result from the fact that the mutant allele is highly temperature sensitive. Penetrance and expressivity were maximized when progeny developed at cool (20°C) temperatures and minimized when they developed at warm (28°) temperatures. Further experiments demonstrated that heterozygotes for the spot allele, which would be mostly normally pigmented "carriers" when raised under warm conditions, would soon begin to develop melanistic spots if transferred to cooler conditions. This is compatible with the hypothesis that the spot phenotype is due to a temperature-sensitive enzyme. © 1983, American Genetic Association.