The within- and among-population variation in individual growth rate of brown trout Salmo trutta L. was studied in five small neighbouring streams (seven isolated populations) within a distance of 70 km in east Norway. Observed growth rate was only weakly correlated with predicted maximum growth rate based on laboratory models, and there was a significant interaction with site. A generalized linear model showed that growth rate was positively correlated with temperature, but also that growth rate decreased as the summer season progressed. This might indicate either a seasonal decline in food availability or appetite, or a change in energy allocation strategy. Growth rate decreased with increasing fish age, probably as an effect of sexual maturation of older fish, and differential allocation of protein and lipid among different size-groups of brown trout. After adjusting for variation in temperature, season, and fish mass, there was still significant among-site variation in growth rate. A significant part of this variation was due to variation in brown trout density and the presence or absence of Alpine bullhead Cottus poecilopus. Growth rate decreased with increasing brown trout density, and was lower in the presence than in the absence of Alpine bullhead after correcting for variation in brown trout density. This last result may indicate the presence of interspecific competition.