A low activity mutant of glucose-6-phosphate dehydrogenase, G6pda-m1Neu has been used to position G6pd in the mouse X chromosome. Linkage tests with tabby, Ta and harlequin, Hq, indicate a likely gene order of Hq–G6pd–Ta. Muscular dystrophy, mdx, has been located by two-and three-point crosses using Hprt, Pgk-1 and Moblo and suggest a gene order of Hprt–mdx–Pgk-1–Moblo. Together with existing linkage data a tentative order for the seven loci is Hq–Hprt–G6pd–mdx–Ta–Pgk-1–Moblo. The relative positions of G6pd and mdx have not been directly tested and G6pd is assigned provisionally proximal to mdx. In the three point test using Hq, G6pd and Ta the recombination frequency found between Hq and Ta was 9·9 ± 2·6%, substantially less than the value of 20·5 ± 2·1% reported by Isaacson et al. (1974).