Otolith microstructure of chinook salmon Oncorhynchus tshawytscha from the Sixes River, Oregon, showed the effects of first-year growth rate on subsequent growth, mortality, and maturation. Although size-selective ocean mortality was not conclusively demonstrated, the results were consistent with the view that smaller fish were removed from the population at a greater rate during oceanic or late estuarine life. On average, fish that were large relative to the rest of the cohort at entry into the estuary remained so up to the formation of the first annulus. Male chinook salmon that grew quickly in the estuary and ocean up to the formation of the first annulus matured earlier, on average.