Otostrongylus circumlitus (Railliet, 1899) from Pacific harbor seals (Phoca vitulina richardsi) and northern elephant seals (Mirounga angustirostris) were examined using morphological and molecular methods to determine whether northern elephant seals along the central California coast are infected by the same species of Otostrongylus as are Pacific harbor seals in the same area. Fixed nematodes were examined and measured using light microscopy. The polymerase chain reaction (PCR) was used to amplify and sequence the second internal transcribed spacer (ITS-2) and D3 expansion (26S) regions of ribosomal DNA of O. circumlitus from Pacific harbor and northern elephant seals. The ITS-2 region was also amplified from Parafilaroides sp. from the Pacific harbor seal, northern elephant seal, and California sea lion (Zalophus californianus) and used for restriction fragment length polymorphism (RFLP) analysis. Morphologically, it was not possible to distinguish O. circumlitus from Pacific harbor and northern elephant seals, and over a consensus length of 443 base pairs (bp) for ITS-2 and 321 bp for D3 the sequences of O. circumlitus from both hosts were identical. With the PCR-RFLP assay, it was possible to distinguish O. circumlitus from Parafilaroides sp. The results suggest that O. circumlitus is the same species in Pacific harbor and northern elephant seals, and molecular methods make it possible to distinguish this nematode from related nematodes.