Egg, embryo, fry, and swim-up stages of steelhead trout (Salmo gairdneri) were exposed to water at total gas saturation levels ranging from 130 to 115%. Eggs, embryos, and newly hatched fry were not affected at 126.7%, but at about day 16 posthatch when the fish began swimming up deaths occurred rapidly, and at the end of the test post button-up mortality ranged from 99% at 126.7% saturation to 45% at 115.3% saturation. Bubbles in the mouth, gill cavity, and yolk sac caused flotation and severe respiratory difficulties. Rupture of yolk-sac membranes also caused death. No differences were noted in survival between fish exposed from egg to fry, and those exposed only from swim-up to fry stage. In summary, there were differences in susceptibility among steelhead life stages to air-supersaturated water; eggs, embryos, and pre-swim-up larvae were more resistant than swim-up and later fry stages.