The goal of this study was to determine the distribution of citrate synthase (CS), -hydroxyacyl CoA dehydrogenase (HOAD) and lactate dehydrogenase (LDH) activities and myoglobin (Mb) concentration in the locomotor muscles (epaxial muscles) and heart of harbor seals. The entire epaxial musculature, which produces most of the power for submerged swimming, was removed and weighed, and three transverse sections (cranial, middle, and caudal) were taken along the muscle bundle. Multiple samples were taken along points on a circular grid using a 6-mm biopsy. A single sample was taken from the left ventricle of the heart. Muscle groups of similar function were taken from three dogs as a control. Mean values were calculated for four roughly equal quadrants in each transverse section of the epaxial muscles. There were no significant differences among the quadrants within any of the transverse sections for the three enzymes or Mb. However, there were significant differences in the mean enzyme activities and Mb concentrations along the length of the muscle. The middle and caudal sections had significantly higher mean levels of CS, LDH, and Mb than the cranial section that may be correlated with power production during swimming. The enzyme ratios CS:HOAD and LDH:CS exhibited no variation within transverse sections or along the length of the epaxial muscles. Relative to the dog, the epaxial muscles and heart of the harbor seal had higher HOAD levels and lower CS:HOAD ratios which, taken together, indicate an increased capacity for aerobic lipid metabolism during diving.