Under the same conditions which yield 1 +4Neµ as the effective number of neutral alleles maintained by fresh mutation at a rate µ at an autosomal locus in a population of size Ne, the corresponding number at an X-linked locus is 1 + 3Neµ. The resultant bimodal frequency distribution for the effective number of alleles does not seem to have been used as the basis for analysis of data on actual numbers of alleles in natural populatio