Three morphological markers (yellow, miniature, and forked) are used to map the location of X-chromosome segments causing male sterility in Drosophila simulans/D. mauritiana and D. simulans/D. sechellia hybrids. In both hybridizations at least three sections of the chromosome contain genes with substantial effects on sterility. This represents the maximum genetic divergence detectable with the three markers, suggesting that the X chromosome contains many loci affecting postzygotic reproductive isolation. The tight linkage between some markers and "sterility loci" may be useful in localizing and later cloning genes important in speciation.