The Apache troutOncorhynchus apachehas become threatened through hybridization with introduced nonnative trouts, among other reasons. We used 10 isozyme locus polymorphisms, which were in the aggregate diagnostic for discrimination of alleles of Apache trout, rainbow troutOncorhynchus mykiss, and cutthroat troutOncorhynchus clarki, to assess extent and directionality of interspecific hybridization in 645 individuals from 31 wild populations within the historical range of Apache trout. Only 11 potentially unhybridized populations of Apache trout were found. Rainbow trout introgression was documented in 19 of the 31 populations, including at least two in which all individuals sampled were hybrids. In four of these introgressed populations, hybridization between cutthroat trout and Apache trout was detected; at two of these localities individuals with alleles from all three species were sampled. Apache-cutthroat hybrids were found at one locality where no rainbow trout alleles were sampled. In 19 of the 20 hybridized populations sampled, a trend of backcrossing toward Apache trout was evidenced. No pure rainbow trout or cutthroat trout were found in the population samples. Because of the extensive hybridization present, it was not possible to estimate the genetic variability extant in the Apache trout genome; only one of seven alleles detected exclusively in hybridized populations could be confidently assigned to a species of origin (rainbow trout). Apache trout recovery efforts will be confounded due to the variable conditions among populations with respect to introgression, habitat deterioration, and barriers to rainbow trout immigration.