Reciprocal group mating crosses were used to investigate the evolutionary status of iso-9 lines of 2 species of the nasuta complex, D. sulfurigaster atbostrigata and D. kohkoa, both collected from Continental Malaysia and Cebu. Assortative mating between iso-9 lines of these and iso-9 lines of 2 other species of the same complex, D. albomicans and D. kepulauana was also investigated using reciprocal group mating crosses. Evidence is presented of a speciation trend between populations of D. sulfurigaster albostrigata from Continental Malaysia and Cebu (Philippines). In contrast populations of D. kohkoa from these same regions show no signs of speciation. Divergence between the mate recognition systems of different species was found to be very high, however hybrids were obtained from some crosses. F1 from D. albomicans and D. kepulauana crosses were found to produce fertile offspring, suggesting a close evolutionary relationship. However, other data show that the mate recognition systems of the 2 species have diverged to such an extent that their specific identity cannot be in doubt.