NADH diaphorase polymorphism was identified in red deer erythrocyte lysates using starch gel electrophoresis and activity staining. The inheritance of the polymorphism was consistent with predictions of autosomal codominant inheritance of two alleles DIA1F and DIAS. In New Zealand's four main feral red deer populations (n = 188) the DIA1F allele frequency ranged from 0.491 to 0.985. A sample of North American wapiti (n = 42) was monomorphic for the DIA1F allele.