Based on personal observations and questionnaires to Norwegian fish farmers, the behavior of three species and several different stocks of salmonid fish reared in ocean pens is described. Swimming activity and the constancy of swimming orientation (clockwise or counter clockwise) vary among species. With few exceptions, the orientation of Atlantic salmon (Salmo salar) at any one farm does not change with tide, season, or age offish. Stock origin and/or the early rearing history may impose this directionality. The swimming of Atlantic salmon in pens resembles schooling behavior. Possible departures from this stereotyped behavior are considered. The behavior of pen-reared salmonids and its implications in selective breeding, migratory mechanisms, and fish health are discussed. Key words: salmon, behavior, pen rearing, mariculture, swimming, orientation, Salmo salar, S. gairdneri, Oncorhynchus gorbuscha