The effects of long-term moderate exercise on the behaviour and the food intake – growth relationship of individually marked Arctic charr, Salvelinus alpinus L., were examined. Direct monitoring of food intake of individual fish was carried out using an X-radiographic method. Growth (weight gain) was significantly improved with increasing swimming speed, and a maximum specific growth rate was obtained at a swimming speed of approximately 1.75 body lengths/s. Growth appeared to be suppressed by high levels of aggressive interactions, and an increase in swimming speed caused a marked increase in schooling behaviour, with a concomitant reduction in agonistic activity. The exercising fish did not consume more food than controls, and moderately exercised Arctic charr showed improved growth – food intake ratios at all levels of food intake compared with fish held in standing water. Furthermore, the proportions of nonfeeding fish were significantly reduced at increasing water velocities. Thus, the relationship between food intake and growth may be strongly influenced by agonistic activity, which appears to be most prevalent in fish subjected to low water velocities.