I studied the age, growth, feeding, and behavior of Sacramento squawfish, Ptychocheilus grandis, in a small stream to provide basic life history information on this species in a habitat where it is often abundant. Young-of-year squawfish in Bear Creek grew throughout the summer and early fall. Mean back-calculated lengths of squawfish ranged from 65 mm SL (standard length) at age I to 257 mm SL at age IV. Mean back-calculated lengths varied among year-classes only at age I, with lengths varying from 55 mm SL for the 1979 year-class to 84 mm SL for the 1977 year-class. Bear Creek contained a large number of small mature fish (ages III and IV), but fish older than age IV were rare. Young squawfish (.ltoreq.100 mm SL) consumed primarily insects and occasionally small fish. Fish became the dominant prey (>50% by weight) at a size of 100 to 150 mm SL. When squawfish were assigned to 50-mm size-classes, diet diversity calculated by size-class (proportions based on the summed contents of all individuals in the size group) was greatest for fish 51 to 100 mm SL. Individual diet diversities (proportions based on individual gut contents) did not vary among size-classes. Stomach fullness of squawfish .ltoreq.100 mm SL declined from May through December 1980. Individual diet diversity of fish .ltoreq.100 mm SL did not vary over the same time period. Large squawfish (100 to 500 mm SL) were sedentary and exhibited little movement within or between pools, during observations of summer, daytime behavior. Large squawfish did not interact aggressively with each or with similar-sized Sacramento sucker, Catostomus occidentalis. A flexible pattern of growth of young-of-year, partitioning of food among life stages, and the sedentary, nonaggressive behavior of large squawfish all contribute to the success of the species in small streams.