Many species retain the physiological capacity to hybridize over long periods of evolutionary time. Hybridization has been reported for sharks and may be the reason for uncertainty regarding phylogenetic relationships within the genus Mustelus. Because of similar life histories and morphologies, limited genetic divergence (both nuclear and mitochondrial DNA), and the degree of uncertainty regarding their phylogenetic relationship, 32 allozyme loci were used to determine the levels of genetic diversity for the Grey Smoothhound, Mustelus californicus, and the Brown Smoothhound, M. henlei, and to detect species-specific nuclear markers capable of determining the existence of hybridization within northeastern Pacific Mustelus. Tissue extracts from 26 adult M. henlei collected from Santa Catalina Island, California and 17 M. californicus collected from Marina Del Rey, California were subjected to starch gel electrophoresis. Gene products of the 32 loci were resolved for all specimens of both species. All 32 loci were monoallelic in M. californicus, whereas four polyallelic loci, three diallelic and one triallelic, were resolved in M. henlei. Nine of the 32 loci exhibited complete allelic divergence between the two species (i.e., no shared alleles) and can serve as diagnostic markers to reveal potential F₁ and F₂ hybrids.