Further studies on the swimming performance of fingerling sockeye salmon at fixed velocities have been conducted in relation to fatigue time. The method of probit analysis, commonly used in dealing with bioassay data, was found to be suitable for determining times to 50% fatigue and in providing a measure of variance despite the presence of some erratic behaviour. For sockeye acclimated to 15 C (mean length = 13.6 cm) the velocity at which 50% fatigued was 54.4 cm/sec or 4.0 lengths/sec (L/sec). The 5% and 95% fatigue velocities were 3.1 and 4.8 L/sec, respectively. Larger fish required longer exposure times for determining maximum sustained speeds, extending from approximately 120 min for fingerlings to 500 min for adults.Using the method of increasing velocity steps the effect of temperatures from 5 to 27.5 C was examined. When acclimated to 15 C fingerling sockeye exhibited only a 4% reduction in swimming speed at 10 and 20 C. Temperatures above the lethal level caused a rapid decline in swimming ability approaching the extinction point at 27.5 C.Recommendations for standard procedures in the study of swimming speeds are made.