Eighteen groups of seventy Atlantic salmon (Salmo salar L.) fry (initial mean body weight 0·8 (SD 0·01) g) were fed on semi-purified diets containing graded levels of L-threonine (Thr) in 15 litres aquaria at a temperature of 14·5^18C. Doses of Thr represented 1, 31, 41, 51, 62, 72, 83 and 93 % of its ideal level for optimum protein deposition. Indispensable amino acids other than Thr were included in the same proportion (on a g/16 g N basis) as in the Atlantic salmon fry whole-body carcass. Following 36 d of feeding and a 36 h fast, fry were killed for whole-body protein and amino acid analysis. Weight gain (r 2 0·98), protein accretion (r 2 0·97), and Thr accretion (r 2 0·97) were linear (P,0·01) functions of Thr intake. Slope of the Thr accretion regression line showed that the efficiency of Thr utilisation above maintenance was 76 %. At zero Thr intake, fry lost 5·4 mg Thr/kg body weight 0·75 per d. The Thr maintenance requirement was 7·2 mg/kg body weight 0·75 per d and the Thr requirement for growth was 66 mg for 1 g protein deposition. Increasing doses of Thr resulted in increased (P,0·05) concentrations of histidine and lysine, and decreased concentrations of isoleucine in whole-body protein. The maintenance need for Thr represented 13·4 % of the total need for Thr. The data suggest that efficiency of Thr utilisation above maintenance is constant at all levels of Thr intake between 1 and 93 % of the level required for optimum protein deposition.