The foraminiferal population of Yaquina Bay was investigated to attempt a correlation of seasonal changes in the hydrography with observed changes in the faunal distribution patterns. The results are based on data from two spatial surveys conducted in July 1966, and late February and March, 1967. The hydrography of Yaquina Bay undergoes seasonal changes closely paralleling the seasonal precipitation pattern. Except for short-term variations, the estuarine system is partly-mixed during the period of maximum river discharge (November to May) and wellmixed from June through October. Coastal upwelling (high salinity, low temperature) has a marked influence on the hydrography of the lower bay during the summer; maximum salinities and temperatures prevail in the upper reaches of the estuary during late summer and fall. Species associations were determined by the Fager-McConnaughey correlation coefficient method; similar assemblages were found for both summer and winter surveys. A marine biofacies fauna, dominated by Elphidium microgranulosum and E. frigidum, occupies the main estuary channel for about five miles upstream. The distributional patterns of the living fauna show evidence of a stronger marine influence during the summer, corresponding to a similar pattern in the hydrography. In the lower bay, the Elphidium fauna is displaced laterally by an assemblage characterized by Trochammina charlottensis and Elphidium incertum in the middle intertidal zone. This assemblages in turn gives way to an Ammobaculites exiguus -Miliammina fusca fauna in the upper intertidal area. The Elphidium assemblage gives way to a Miliammina fauna in the upstream direction. Trochammina inflata, T. macrescens, Miliammina fusca, Jadamminina polystoma, Haplophragmoides sp., H. hancocki and Siphotrochammina lobata are the principal species found in the marine marsh. Zones of environmental similarity were defined by the index of affinity association analysis. Living and total faunas of both winter and summer surveys were considered separately; several sample groups were defined in each case. Index of affinity data can also be used to provide an index of the marine influence on the foraminiferal population of the estuary. Species diversity or the average number of species/sample, ranges from 22 in the marine biofacies zone to six on the lower bay tideflats. An intermediate value (l0/sample)was found for the marine marsh. Highest standing crops were found in the marshes (summer) and on the tideflats adjacent to the lower bay channel, averaging 40 and 42/cm² respectively. Lowest values (1 to 2 cm²) were found in the middle to upper intertidal areas of the lower bay during the summer. The distribution of the total fauna is quite similar to that of the living population. Highest average values (175/cm³) are again found in the marine marsh and in the marine biofacies zone (70 to 80/cm³). Lowest values occur on the lower bay intertidal areas (3 to 9/cm³). In general, the percentage representation of agglutinated species in the total fauna increases with distance upstream in the channel and with distance laterally from the channel axis. Faunas of some marsh stations are composed entirely of agglutinated species. The percentage of calcareous hyaline species shows an inverse relation to that of the agglutinated fauna, increasing in importance with approach to marine conditions. Porcelaneous and planktonic species are generally minor constituents in the total fauna and are restricted to the marine biofacies as it is developed in the estuary. Foraminifer-ostracod ratios are extremely variable in the environments sampled in Yaquina Bay. Smooth-valved species are characteristic of the marine marsh and inner bay environments; valve ornamentation tends to increase with approach to marine conditions. Thecamoebians are present in low numbers throughout the estuary. In general however, the number of species and population size increases in the upstream direction. / Graduation date: 1968