Dobzhansky and Spassky ( 1959) and Dobzhansky, Ehrman, Pavlovsky, and Spassky ( 1964) found that Drosophila paulistorum is a superspecies composed of several semispecies.The semispecies show so strong an ethological isolation from each other that in some places they coexist sympatrically without interbreeding.Moreover, among the hybrids between the semispecies produced in the laboratory, the females are fertile while males are completely sterile.Originally, six semispecies were recognized-the Centroamerican, Orinocan, Amazonian, Andean-Brazilian, Transitional, and Guianan.More recently, the Guianan was raised to a full-fledged species, Drosophila pavlovskiana (Kastritsis and Dobzhansky, 1966).The Transitional was so named because certain strains from Colombia were found to cross and to produce fertile hybrids with some Centroamerican as well as some Andean-Brazilian strains.Cytological studies of Kastritsis ( 1966, 196 7), as well as our own genetic data, led us however to conclude that some of the strains originally classified as Transitional were in reality extreme northern representatives of the Andean-Brazilian semispecies.Meanwhile, the accession of freshly collected strains from western and northern Colombia and northwestern Venezuela forces us to redefine the Transitional race (or semispecies) , as will be described in the report that follows.The Transitional so redefined includes only a part of the strains formerly included under that name.A wholly unexpected development has been a change in the behavior of one of the strains; this strain was originally classified