Cytogenetic analysis of more than 1500 randomly recovered lethal X chromosomes derived from 2000 and 3000 r X-ray exposures of post-meiotic male germ cells has made possible a plot of the distribution in different regions of the X chromosome of: (1) gene mutations associated with cytologically normal chromosomes, (2) mutations associated with chromosomal rearrangement breakpoints, (3) deficiencies, and (4) rearrangement breakpoints whether or not they are associated with mutations. The distribution of point mutations, vital loci and rearrangement breakpoints in different regions of the X chromosome is not proportional to either the number of bands or the relative DNA content. Further, the density of vital loci (those capable of mutating to a lethal allele) is quite different in some regions as compared to others. For example, vital loci in the 3AB region, which has been thoroughly studied by Judd and others, are at least as numerous as bands; whereas, the 3CD region, equally long, has only two vital loci. Other regions densely populated with vital loci include 1B, 1F-2A, 10A, 11A, and 19EF; sparsely populated regions include 6EF and 10B-10E. It seems reasonable to conclude that the recovered X-ray-induced mutants available for analysis do not represent a random sample of those initially induced in the exposed male germ cells.