In an attempt further to test Berg’s inference that there are “much more numerous sterility mutations in theX-chromosome than in the second chromosome and possibly more than in all the autosomes taken together”, experiments were devised to find the frequency of the “male-specific”. (producing sterility in the males only, leaving the females fertile) and “female-specific” (producing sterility in the females only, leaving the males fertile) dominant sterility mutations produced by X-rays in the autosomes, as these had not been studied by Berg. The results obtained show that such sex-specific dominant sterility mutations must be very rare, as compared with recessive sterility mutations of all kinds. Berg’s original conclusion, therefore, is confirmed. Consideration of the data on sterility of the different classes ofF 1 andF 2 males and females shows that the rather high percentage of sterility among the male offspring of irradiated males is due, in very large measure, to absence or incompleteness of theY-chromosome in the affectedF 1 males. By far the greater part of the sterility of the grandsons of irradiated males is due to sterility genes located in theX-chromosome. Localization of eight of theseX-steriles shows at least five, and possibly as many as seven of them, to be located in the relatively short region between garnet and forked. This lends support to a suggestion made by Berg that not merely theX-chromosome as a whole, but its active region, contains per unit of length a greater number of genes affecting sex than do the other chromosomes, and it is in agreement with data by Berg & Kovalev dealing with the location of seventeenX-steriles. The present data, however, limit considerably more than those heretofore obtained the extent of the active region chiefly concerned in theX-chromosome sterility mutations, and suggest that most of them must be confined to a region not over ten linkage units long, and perhaps even to a single locus. This transfers a large part of the problem involved in the high frequency ofX-chromosome sterility mutations (and perhaps much of the problem of sex determination also) from theX-chromosome as a whole to this small region, or locus.