Abstract.Two studies were conducted to determine the effects of rearing Snake River cutthroat trout,Oncorhynchus clarkiRichardson, and Eagle Lake rainbow trout,Oncorhynchus mykissRichardson, in oxygen supersaturated water. The performance of cutthroat trout held at oxygen saturation as high as 172% was compared with that of control fish held in water at or below saturation. At an oxygen saturation level of 172%, total gas pressure reached 117% in late afternoon, and nitrogen saturation was reduced to 93%. The rainbow trout were held at a maximum of 150% oxygen saturation; total gas pressure did not exceed 112% in late afternoon and nitrogen saturation was reduced to 99%. Fish growth, fin quality and feed conversions were not significantly affected in either species. At the termination of the study gas bubble disease was observed in 94% of the cutthroat trout held in oxygen supersaturated water. Gas bubble disease was not observed in rainbow trout.