SUMMARYBare-patches (Bpa), a semi-dominant, sex-linked, male-lethal gene in the mouse is described. It shows 14·5 ± 3·0% recombination with Blo and 5·4 ± 2·2 % with Ta and may, therefore, be allelic with Str.Only about half the expected number of heterozygous females is observed; the remainder appear to lose their mutant chromosome and become XO females as there is an excess of non-Bpa females, at least 24% of which proved to be XO. It is not yet certain whether the lost X chromosome is always the mutant one nor is it certain whether the loss is a property of the Bpa gene or due to a closely linked factor: a separate factor seems more likely. Some possible mechanisms for XO production are discussed.