Development of hypo-osmoregulatory ability, gill Na+,K +-ATPase activity, condition factor and growth in Atlantic salmon during parr-smolt transformation was studied in a 2??3 factorial design with three temperatures (12.0, 8.9??C and ambient, 2.4-11.9??C, mean: 6.0??C) and two farmed strains of smolts (Mowi and AquaGen). The development of hypo-osmoregulatory ability and gill Na+,K+-ATPase activity were significantly influenced by freshwater temperature. In smolts raised at 12.0??C, maximum gill Na+,K+-ATPase activity was reached in late April, compared with late May and mid-June in the 8.9??C and ambient groups, respectively. In all groups, peak gill Na+,K +-ATPase activity was seen 350 degree days (d??C) after the onset of the smolt-related increase in enzyme activity (30 March) The period of high enzyme activity (>90% of maximum) lasted approximately 250 d??C. No distinct peak level in gill Na+,K+-ATPase activity was seen in the AquaGen strain at ambient temperature. Elevated temperatures also accelerated the loss of hypo-osmoregulatory capacity. In all groups, gill Na+,K+-ATPase activity reached pre-smolt levels approximately 500 d??C after the calculated peak level. Growth rate in freshwater was influenced by strain, temperature and their interaction, with the Mowi strain showing a higher growth rate than the AquaGen strain at 8.9??C and ambient temperatures. Following transfer to seawater, a higher growth rate was recorded in smolts from the Mowi strain than the AquaGen strain from the ambient temperature regime. Temperature influences the development and loss of smolt characteristics in both strains, and has long-term effects on post-smolt performance in seawater. ?? 2004 Elsevier B.V. All rights reserved.