Twenty-nine members of a black kindred with congenital X-linked incomplete achromatopsia were examined; nine affected males and seven carrier females were identified. The new findings of importance are the following: (1) this congenital disorder is a slowly progressive abiotrophy, with progressive macular scarring and cone dysfunction, rather than a stationary anomaly; (2) carrier females sometimes can be found by ophthalmoscopic and fluorescein angiographic abnormalities in the macula; and (3) genetic linkage studies give evidence against linkage with the locus for the Xg blood group, but they suggest possible linkage with the glucose-6-phosphate dehydrogenase locus.