Burst swimming by newly emerged coho salmon from two populations was studied with electrical shock stimulation and high-speed cinematography. At emergence, Morrison Creek fish were larger (31 versus 27 mm standard length) than fish from Wade Creek. The larger fish attained greater mean and maximum burst speeds than the smaller fish and differences in swimming performance (up to 80%) were greatest over the first 0.03 s of acceleration. When exposed to 140-mm coho salmon predators, more of the larger Morrison Creek fish survived than of the Wade Creek fish. Size-mediated differences in burst swimming performance appear to be the basis for the observed difference in susceptibility to predation.