Drosophila simulans and D. mauritiana are sibling species, the former cosmopolitan and the latter restricted to the oceanic island of Mauritius. Sex comb-tooth number of male flies of D. simulans were about 9.83, while those of D. mauritiana were 12.90. Genital arch of D. simulans is large semicircular shaped expasion, while that of D. mauritiana is a narrow fingerlike expansion. We used classical genetic analysis to measure effects of genes on the X chromosome responsible for numeral and morphological differences in sex comb-tooth and genital arch between these species, respectively. For these purposes, mutant strain of D. simulans and wild type strain of D. mauritiana were hybridized and males of the FI and the backcrossed progenies were compared with two characters above mentioned. The sex comb-tooth number of F, males were about 11.79, and the genitalia of F, male were intermediate in shape between those of D. simulans and D. mauritiana. Genetic analysis of sex comb-tooth number and genital arches differing between D. simulans and D. mauritiana showed that very little diffemce was due to effect of the X chromosome.