Four species belonging to the Drosophila melanogaster complex were examined genetically and morphologically to analyze interspecific relationships. Insemination rates ranged from 96% to 99% within species crosses, but interspecific crosses among the four species exhibited a great variations in the frequency of successful matings. D. melanogaster females mated relatively well with males of other species and D. sechellia males were more successful in mating with females of other species. In the crosses among D. simulans, D. mauritiana and D. sechellia, hybrid flies were fertile in females, but sterile in males regardless of reciprocal matings. The phenogenetically relationship between this complex and their hybrids were investigated by the comparison of sex comb tooth number and genital arch of male. They were controlled by polygenic factors on the chromosome of both parents. The effects of temperature on viability of hybrids between D. melanogaster females and D. simulans males were investigated for detection of genes concerning the speciation. The temperature sensitivity of the hybrid was mainly controlled by genes located on the X chromosome of D. simulans males.