Image analysis was used to quantify the activity of a temperature-sensitive macromelanophore-determining allele in sailfin mollies as the percentage of the body surface area covered by macromelanophores. Fish heterozygous for the macromelanophore-determining allele produced very few macromelanophores when raised at either 25 or 28°C, even after more than 200 days. In contrast, the mean percent coverage for genetically identical fish raised at 22°C increased steadily throughout the course of the experiment. Production of macromelanophores was sex influenced, with greater expressivity seen in males. At 22°C, the mean percent coverages had significantly diverged between males and females by the age of 201 days. From that point on, the percent macromelanophore coverage of the males was consistently significantly higher than that of the females. The tendency to produce greater melanization at cooler temperatures is not the result of a heat-sensitive tyrosinase enzyme, as is the case in mammals carrying the Himalayan allele. In mollies, the activity of tyrosinase increases between 22 and 29°C. We hypothesize that production of macromelanophores is under the control of a proto-oncogene. Copyright © Munksgaard, 1999.