Introduction In previous work it has been shown that when a spotted platyfish ( Platy-poecilus maculatus ) is mated with a Mexican swordtail ( Xiphophorus hellerii ) melanosis develops in all those hybrids which inherit the spotting factor from their platyfish parent. When the platyfish parent is homozygous for the spotted factor ( SpSp ), melanosis develops in all the hybrids; whereas if the platyfish parent is heterozygous for the spotted factor ( Spsp ), one-half of the hybrids are melanotic and the other half are normal. The platyfish has two types of melanophores—one type, the smaller, called micromelanophores, is inherited as an autosomal dominant, whereas the large type, called macromelanophores, is inherited as a sex-linked dominant factor (Gordon 1927). Both types may be, and usually are, present in a single individual, though genetic strains have been developed in which only one or the other type of melanophore is present. A number of different hybrid combinations are possible between the platyfish and the swordtail with reference to their melanophore content. The combinations are listed below, together with the incidence of melanosis in the particular type of hybrid. It is evident from the above table and from Fig. 1 that, only when the platyfish parent transmits macromelanophores to the hybrid offspring, do these offspring develop melanosis. Platyfish may have micro- and macromelanophores, but macromelanophores alone combined with the hereditary factors of the swordtail are sufficient to bring melanosis to their hybrid young. In all the cases listed in the table and figure, the swordtail parent had micro-melanophores. The following experiments describe the results when sword-tails without melanophores are used as parents for hybrid offspring.