The tolerance of sac fry of lake trout, Salvelinus namaycush (Walbaum), to the acute effects of gas supersaturation, from hatching to swim-up, was tested using six gas levels, ranging from ΔP 8 to 148. Although many fish were moribund by the time the yolk was nearly absorbed, mortality during the 40-day study was negligible; survival to swim-up was 96–99%, including 99% survival at ΔP 148. Of fish examined at ΔP 42, 40% showed bubble formation around the rim of the eye. Signs of trauma were greatest at ΔP 119 and 148, and included intestinal bubbles, a distended abdomen, and bubbles around the eye, in the mouth and in jaw tissues.