A hypervariable DNA marker is closely linked to one of the most severe forms of night blindness, X-linked retinitis pigmentosa (RP). Affected individuals with X-linked RP, obligate carriers, and ophthalmologically identifiable carriers of the disease were included in a linkage study. The diagnosis was established in five sibships by funduscopic and electrophysiological investigations. When the X-linked probe M27 beta was used, 2 recombinants out of 29 informative meioses were detected (theta = 0.07 at a maximum lod of 4.75). The hypervariable probe detected two different alleles in 38 of 39 females tested. M27 beta is therefore a potentially very useful probe for carrier detection and prenatal diagnosis, as well as for addressing the question of heterogeneity of X-linked RP.