Twenty-three bottom-trawl fish assemblages were identified from the relative biomass of 33 dominant species that occurred in the National Marine Fisheries Services' triennial trawl surveys over the continental shelf and upper slope off California, Oregon, and Washington from 1977 to 1992. The assemblages accounted for about 70% of the total variation in species composition among 2,565 hauls. Although the assemblages persisted over the 15-yr study period and occurred within broad geographic boundaries, some had substantially different spatial distributions among surveys. The ability to differentiate assemblages across five environmental variables (latitude, depth, surface and bottom water temperatures, and surficial substrate) was low. The preponderance of hake-dominated assemblages throughout the study area suggests that Pacific hake (Merluccius productus) may play a large role in the dynamics of demersal fish communities off the west coast of the United States. The same data were used to establish general features regarding the abundance and distribution of the 33 dominant fish, and investigate intersurvey regional variation in species composition. Flatfish were generally widespread and at low density within areas of their occurrence. In contrast, rockfish were comparatively less widespread, and were at higher density within areas of their occurrence. Pacific hake, spiny dogfish, and sablefish were both widespread and occurred in high density. The greatest amount of variation in species composition occurred in the shallow shelf region off California, and the shallow and deep regions between Cape Mendocino and Cape Blanco. These regions corresponded to areas with the greatest amount of annual variation in upwelling. Contrary to upwelling, intersurvey variation in surface temperature did not appear to correspond to variation in species composition, but there was an unanticipated negative correlation between variation in bottom temperature and variation in species composition. Species composition was influenced in most regions by Pacific hake, spiny dogfish, and sablefish. A conceptual model was developed to explore the relationship between regional changes in a species' biomass, incidence, and density, and their potential affect on species composition. Empirical examination of the model was difficult. Five of six flatfish species (Pacific sanddab, rex sole, Pacific halibut, Dover sole, and English sole) exhibited a significant positive linear relationship between incidence and log-transformed biomass which is consistent with density-dependent habitat selection. There was evidence (albeit weak) from patterns in the occupancy of substrate types by these flatfish, that marginal habitats are associated with areas of mud for Pacific sanddab and areas of sand for rex sole, Pacific halibut, and Dover sole. / Graduation date: 1997