The reproductive success of many salmonid species relies on successful migration to natal spawning grounds. Energy efficient locomotion, dictated by cost of transport (COT), may be an important selection pressure for completion of migratory routes. In addition, climate change poses further difficulties for migration due to elevated temperatures and varying water velocities. To explore how temperature affects efficiency of locomotion, rainbow trout (Oncorhynchus mykiss) were subjected to a modified critical swimming speed (UCrit) test, while oxygen consumption (MO2) and swimming form were recorded, at acclimation (11°C) and high (20°C) temperature. Calculation of strouhal numbers (St) enabled gait change to be assessed in terms of swimming efficiency and performance. Increasing temperature from 11-20°C gave a large increase in oxygen consumption (Q10 = 1.67). Incremental increase in swimming speed (from 1 Bl s-1 to UCrit) resulted in further increases in oxygen consumption (349.942 ± 53.02 and 423.185 ± 51.5 1mg O2 kg-1 h-1 for controls and high temperature, respectively). However, the rate of MO2 increase with increasing water velocity was not different between the two temperature groups, or at different gaits. As previously shown, tail beat frequency and amplitude increased to accommodate greater speed, up to the onset of burst and glide gait, but at high speeds tail beat amplitude alone was modulated. Rainbow trout swum within the strouhal number range for optimum propulsive efficiency (0.2 < St < 0.4), as seen in numerous animals. Increased velocity and gait change did not affect strouhal numbers at low temperature. However, at high temperature strouhal number increased with speed, due to an augmented tail beat frequency at high temperature. This result suggests a higher muscle twitch rate at high temperature, in agreement with previous muscle physiology data.