The inactive-X hypothesis (Lyon, 1961) states that in the normal female mouse only one of the two X chromosomes is genetically active in each cell of the body other than the germ-cells, and that the choice of which X is to be inactivated occurs at an early stage of embryogenesis, and is at random in each cell. The descendants of these cells then abide by the decision so that females are mosaics for two lines of cells and may show a mosaic phenotype if they are heterozygous for a sex-linked gene. One requisite for the validity of this hypothesis is that only one X is necessary for the development of a normal female mouse, and evidence substantiating this supposition is the apparent normality of XO mice (Welshons & Russell, 1959; Cattanach, 1961a, b). On the other hand it has been suggested that XO mice are not fully viable (Russell, Russell & Gower, 1959), but no data on the comparative viability and growth of XO female mice have yet been reported. In the present communication data of this nature, collected in the process of setting up an XO stock, are presented and support the hypothesis that only one X chromosome is necessary for normal female mouse development.