This study examined how acute warming of water temperature affects the mechanical efficiency of swimming and aerobic capabilities of the brown trout Salmo trutta. Swimming efficiency was assessed using the relationship between swimming kinematics and forward speed (U), which is thought to converge upon an optimum range of a dimensionless parameter, the Strouhal number (St). Swim-tunnel intermittent stopped-flow respirometry was used to record kinematics and measure oxygen consumption ( ˙M O2) of S. trutta during warming and swimming challenges. Salmo trutta maintained St between 0⋅2 and 0⋅3 at any given U over a range of temperatures, irrespective of body size. The maintenance of St within the range for maximum efficiency for oscillatory propulsion was achieved through an increase in tail-beat frequency (f tail) and a decrease in tail-beat amplitude (A) as temperature increased. Maintenance of efficient steady-state swimming was fuelled by aerobic metabolism, which increased as temperature increased up to 18∘ C but declined above this temperature, decreasing the apparent metabolic scope. As St was maintained over the full range of temperatures whilst metabolic scope was not, the results may suggest energetic trade-offs at any given U at temperatures above thermal optima.