We used 51 isozyme loci to determine the population genetic structure of bull trout (Salvelinus confluentus) in the Columbia and Klamath River drainages. The sampled populations have little genetic variation within and significant genetic differences among themselves. Preserving the genetic diversity of bull trout will require the continued existence of many populations throughout this region Bull trout from the Columbia and Klamath drainages would qualify as separate ''species'' under the United States Endangered Species Act according to criteria established for anadromous salmonid fishes. Genotype frequencies at the four variable loci in a group of bull trout used for artificial supplementation indicate an extremely small number of effective parents. The release of such fish into the wild could have harmful effects on native fish populations. Non-native brook trout (Salvelinus fontinalis) have been introduced throughout the range of bull trout, and hybridization between these species has been reported Protein and mitochondrial DNA genotypes collected from one stream in western Montana over an eight-year period indicate the displacement of bull trout by brook trout. This rapid displacement was accompanied by extensive production of interspecific first-generation hybrids by both reciprocal crosses.
