We used radiolabeled DNA of bacteriophage M13 to probe Southern blots of restriction enzyme-digested genomic DNA of rainbow troutOncorhynchus mykiss. We found a high level of polymorphism for restriction fragment lengths by analysis of DNA from a single-pair mating between individuals of an outbred strain of this species. Each of the offspring of this mating exhibited a unique banding pattern. Similar polymorphic patterns were found in chum salmonO. keta, coho salmonO. kisutch, and Atlantic salmonSalmo salar. This technique shows great promise for demographic analysis and unabiguous genetic tagging of individual fish.