THE red wolf, previously endemic to the southeastern United States, declined precipitously in numbers after 1900 because of habitat destruction, predator control programmes, and hybridiz-ation with coyotes1,2. Hybridization with coyotes probably occurred as these animals, which adjust well to agriculture, became numerous and moved eastwards1-4. By 1970, red wolves existed only in extreme southeastern Texas and southwestern Louisana (Fig. 1)2. In 1967, red wolves were classified as endangered and a captive breeding programme was begun in 1974 after passage of the Endangered Species Act, about a year before they became extinct in the wild. Protein electrophoresis and morphometrics have been used to try to discriminate red wolves from hybrids and coyotes1,4,5. But because the average substitution rate of mitochondrial DNA in mammals is much greater than that of nuclear genes6, mtDNA analysis is a more useful way of distinguishing closely related species. We have now analysed mtDNA restriction-enzyme sites and cytochrome b gene sequence variation in captive red wolves and in 77 canids sampled during the capture period. We also used the polymerase chain reaction to amplify and then sequenced mtDNA from red wolf skins collected before substantial hybridization of red wolves with coyotes is thought to have occurred. Phylogenetic analysis indicates that red wolves have either a grey wolf or coyote mtDNA genotype, demonstrating hybridization among these species. Thus, the red wolf is entirely a hybrid form or a distinct taxon that hybridized with coyotes and grey wolves over much of its previous geographical range. Our findings, however, do not argue against the continued protection of the red wolf.