THE Amazon molly, Poecilia formosa (Girard), is an all-female species native to southern Texas and north-eastern Mexico1. In the northern part of the range, P. formosa is fertilized by males of P. latipinna; in the southern part by a species of the P. sphenops complex. Reproduction is usually by gynogenesis; no introgression of paternal characters into P. formosa stocks has been observed in any of the several laboratories in which they have been maintained during the past 36 yr1–7. With no genetic material being transferred from the male parent to the offspring, all P. formosa which have descended from a single progenitor should possess the same genotype and constitute a clone. The existence of several clones in natural populations has been demonstrated by tissue transplantation4,5. In rare instances some offspring of P. formosa females exhibit paternal characters and thus are hybrids6,8,9. In the Genetics Laboratory of the New York Aquarium where several clones of P. formosa have been maintained since 1960 by either mating them to males of P. vittata or P. sphenops (black molly stock), eighteen hybrids (about 1 per cent) have been obtained. Of these, three were sired by P. vittata and fifteen by P. sphenops.