The age-distribution in the late-onset form of melanomas in the Xiphophorus fish hybrids was determined. The domesticated stock of the platyfish carrying the macromelanophore gene, Sp, was hybridized with the green swordtail or albino swordtail to produce F1 hybrids. The hybrid fish developed pigmentary abnormalities, melanosis, at early ages. These fish matured sexually at about 6 to 8 months of age, although many of them were sterile. The senescence pattern of the hybrid fish was estimated on the basis of survival curves. In the hybrids between the platyfish and the green swordtails, the 90%, 50%, and 10% survival points were 12, 16, and 19 months, respectively. The cumulative percentage of the fish developing melanomas increased sharply in proportion to the third power of the age from 5 to 19 months. The cumulative percentage at 19 months of age was 22.2%. A similar age-dependence of melanomas was also found in the hybrids between the platyfish and the albino swordtail, although melanomas in this population occurred at an earlier age with higher cumulative percentages. The cumulative percentage of this population was 53.6% at 20 months of age.