1. A regular annual growth-rate cycle is demonstrated in wild and hatchery yearling brown trout; the fish have a high growth rate in the spring and autumn and a low growth rate during the summer and winter of each year.
2. Experimental work with constant-environment aquaria, together with the results of the field work, indicate that the water temperature is the main external environmental factor influencing the growth rate. Maximum growth rate is achieved at 12° C.
3. The reason for the fall in growth rate above 12° C. is discussed and it is suggested that inadequacy of the respiratory system of the fish is the prime cause.