SUMMARY There is strong evidence that pinnipeds maintain a lipid-based, aerobic metabolism during diving. However, the few fiber-typing studies performed on pinniped skeletal muscles are not consistent with an aerobic physiological profile. The objective of this study was to reexamine the fiber type distribution throughout the primary locomotory muscles of the harbor seal Phoca vitulina . Results from immunohistochemical (IHC) fiber typing indicated that harbor seal swimming muscles (the epaxial muscles) are composed of 47.4% type I (slow twitch, oxidative) fibers and 52.8% IIa (fast twitch,oxidative) fibers, which are homogeneously distributed throughout the muscle. Harbor seal pectoralis, a secondary swimming muscle, was composed of 16.2%type I and 84.3% type IIa fibers. No fast twitch, glycolytic (type IIb) fibers were detected in either muscle, in contrast to published data on fiber typing of harbor seal epaxial muscles using traditional histochemical techniques. The extreme specificity inherent in the IHC fiber typing procedure leads us to conclude that harbor seal swimming muscle is entirely composed of oxidative fibers. Our results are consistent with the enzymatic analyses of pinniped skeletal muscle that support the use of lipid-derived aerobic catabolism to fuel working muscle during diving in these marine mammals.