D.n. nasuta, D.n. albomicans, Cytorace I and Cytorace II [Chromosoma, 93 (1986) 243] are closely related strains of the nasuta subgroup. Through male choice and female choice experiments, the pattern of sexual isolation if any, among them was analysed. Differential mating preference of males and females of these races suggests that they are passing through the process of anagenesis. Therefore, this, as an evolutionary experiment under laboratory conditions, offers a rare opportunity to witness different pattern and levels of divergence among four cytogentically parsimonious races of Drosophila.