Although there is no evidence of hybridization in nature, interspecific crosses in all six possible pairwise combinations may be effected in the laboratory amongst the four species of the Drosophila bipectinata complex (D. bipectinata, D. parabipectinata, D. malerkotliana and D. pseudoananassae) of the Oriental-Australian biogeographic zones. Results range from a high degree of crossability in one direction with production of large numbers of sterile male and fertile female offspring in a 1 : 1 ratio (bipectinata x parabipectinata) to almost complete isolation in both directions; in the latter case (malerkotlianaxpseudoananassae) the very small proportion of successful crosses yielded only a few females oflow viability. These results combined with those previously obtained in polytene chromosomal and electrophoretic investigations suggest very close phylogenetic relationships among bipectinata, parabipectinata and malerkotliana, with a greater degree of divergence between these species and pseudoananassae.