The Hemigrapsus oregonensis population at Coquille Point in the Yaquina Bay Estuary on the Central Oregon Coast was studied from April, 1972 through May, 1973. The population was found to be vertically stratified from the 1 ft level to the 5 ft level. Population densities were found to be most dense in the upper regions. Greatest population density (about 20 crab/m²) was found to be in the 3-4 ft interval above MLLW (0.0 ft level). The population sex ratio was biased in favor of the females (53.3%) and did not vary appreciably during the year. The reproductive season, as determined by the percentage of berried females, was from February through May with a peak (32.8%) during March. Brooding females were found every month during the study, indicating a continuous, low level egg production throughout the year. A model for estimating potential egg production is given. The minimum carapace width of brooding females was found to be 0.86 cm. Biomass values were determined from carapace width measurements. A conversion equation is given. Biomass values generally increased as tidal height increased. The average biomass value for the area was 8.47 g/m². The average dry weight per crab decreased as tidal height increased, The average dry weight per crab at each tidal height (about 0.5 g) did not significantly increase during the study, suggesting a stable population. The average monthly production showed an over-all negative rate of -1.23 g/m² per month. No significant differences were found between tidal heights. The net production rate at each tidal height could not be shown to be different from a zero net production rate, again suggesting a stable population. Monthly distributional patterns indicated an high degree of population mobility. Crabs tested for locomotory activity patterns in the laboratory showed rhythms influenced by both the light regime and the tidal regime. Weak endogenous displays were found for a light component with increased activity during dark periods. Greatest activity generally occurred during dark-high tide periods. It is suggested that the locomotory activity patterns of H. oregonensis are influenced by both a tidal cycle and a light cycle. Under constant experimental conditions, the endogenous rhythmicity decayed within 3-9 tidal cycles and resulted in more or less continuous random movements. Only about 50% of the tested crabs, however, displayed an endogenous locomotory rhythm. / Graduation date: 1974