The polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique was used to identify 3 species in the genus Opisthonema (Opisthonema bulleri, Opisthonema medirastre, and Opisthonema libertate) in the southern Gulf of California. Two specific primer pairs were designed to amplify the COI mitochondrial gene and 3 restriction enzymes (HpaII, RsaI, and AvaII) were selected to compare the RFLP patterns among the species. Different polymorphism levels were detected among samples. Easily analyzed specific restriction profiles were created, which differentiated unequivocally among the 3 target species. The approach undertaken here using PCR-RFLP is relatively cheap, fast, and robust, providing a useful technique for the identification of species in the genus Opisthonema. [ABSTRACT FROM AUTHOR]