Physical environment and physiological characteristics of marine mammals potentially affect the duration and depth of diving. Härkönen (1987b) proposed a hypothesis that the harbor seal would gain maximum energy by foraging at intermediate depths. To investigate this hypothesis, we studied diving behavior of the Pacific harbor seal (Phoca vitulina ricbardii) during 1995 through 1997 in Monterey Bay, California. Dive depths (n = 13,063 dives) were recorded via time-depth recorders. Approximately 80% of recorded dives were classified as square dives (type I), which typically were associated with foraging in pinnipeds. Approximately 11% of dives were V dives (type II; 1,402 dives), and the remainder (1,225 dives) were skewed dives (type III and IV). The deepest recorded dive was 481 m, while the greatest duration was 35.25 min. Body mass explained the variability of durations of long dives for females (95th percentile; D95♂=-5.47 + 0.18 × (mass♀), r2= 0.91, 95% CI for slope = [0.08, 0.28], n= 5) and for males (D95♂=-5.86 + 0.18 × (mass♀), r2= 0.83, 95% CI for slope = [0.12, 0.24], n= 11). The large proportion of variability in deep dives, however, was explained by body mass only for males (95th percentile; Z95♂=-363.9 + 6.05 × (mass♀), r2= 0.83, 95% CI for slope = [3.93, 8.17], n= 11) and not for females (Z95,♂=−148.1 +3.11 × (mass♀), r2= 0.58, 95% CI for slope = [-1.7, 7.9], n= 5, 95% CI for slope= [−1.7, 7.9]). Median depths of presumed foraging dives of harbor seals in the Monterey Bay area were between 5 and 100 m, which were within the range of the previously reported depths for other areas (< 100 m). Our findings generally supported Härkönen's hypothesis that harbor seals forage in the intermediate depth in their environment.