Growth rates of Atlantic cod vary both between populations in the same year and within a population over a series of years. The latter type of change is selected for detailed study, using an example found in the population resident in the western Gulf of St. Lawrence (ICNAF Division 4T). A real change in growth is verified by examining ageing methods, selectivity of sampling, tagging returns, and by measuring increments on the otoliths. Laboratory experiments showed that, when food supply is unlimited, food consumption increases with increasing temperature, and that growth increases with increasing food consumption. Direct correlation between temperature and growth changes is not established.Cod growth changes in Division 4T are found to be the result of changes in abundance and availability of herring as food, plus changes in competition for food resulting from density changes in fish-eating members of the cod population.