A new subspecies, Drosophila tropicalis cubana, belonging to the siblings of the willistoni group of the subgenus Sophophora and distributed throughout the Greater Antilles is described and compared with the South American subspecies, Drosophila tropicalis tropicalis Burla and da Cunha. The two subspecies overlap widely in the ranges of values for certain morphological traits conventionally used in describing species of Drosophila; although some of the means differ significantly, not one of the traits can be used for separating the subspecies with any degree of reliability. Therefore, the morphological differences are cryptic. Sexual isolation between the subspecies is minor, if it exists at all, and confined to one of the reciprocal intercrosses, both of which produce viable hybrids-fertile females that are heterotic as shown by increased fecundity and completely sterile males that have testes containing degenerating spermatids. At present the subspecies are best distinguished by intercrossing with known...