ABSTRACT: In this study, experiments were conducted to examine the effect of an acute necrotic bacterial gill infection on the metabolic rate (M_O2) of Atlantic salmon Salmo salar. Fed and unfed Atlantic salmon smolts were exposed to a high concentration (5 × 10¹² CFU ml^–1) of the bacteria Tenacibaculum maritimum, their routine and maximum metabolic rates (M_O2rout and M_O2max, respectively)were measured, and relative metabolic scope determined. A significant decrease in metabolic scope was found for both fed and unfed infected groups. Fed infected fish had a mean ± standard error of the mean (SEM) decrease of 2.21 ± 0.97 µM O₂ g^–1 h^–1, whilst unfed fish a mean ± SEM decrease of 3.16 ± 1.29 µM O₂ g^–1 h^–1. The decrease in metabolic scope was a result of significantly increased M_O2rout of both fed and unfed infected salmon. Fed infected fish had a mean ± SEM increase in M_O2rout of 1.86 ± 0.66 µM O₂ g^–1 h^–1, whilst unfed infected fish had a mean ± SEM increase of 2.16 ± 0.72 µM O₂ g^–1 h^–1. Interestingly, all groups maintained M_O2max regardless of infection status. Increases in M_O2rout corresponded to a significant increase in blood plasma osmolality. A decrease in metabolic scope has implications for how individuals allocate energy; fish with smaller metabolic scope will have less energy to allocate to functions such as growth, reproduction and immune response, which may adversely affect the efficiency of fish growth.