Abstract 1. 1. Fish mortality due to low-level nitrogen supersaturation in natural waters (as little as 15 per cent) raises the question of their sensitivity to decompression-induced diver's bends. 2. 2. Fingerling salmonids ( Salmo gairdnerii and Onchorhynchus kisutch ) have been saturated with air, and normoxic helium and argon at a range of initial pressures ( P i ), ∗ up to 6 atm, and then decompressed through a range of initial/final pressure ( P f ) ratios ( P i / P f ) as high as 8.0. 3. 3. The results clearly show marked differences in bends-producing potential of these different gases with formation of histologically demonstrated tissue bubbles and emphasize the unsuitability of either excess partial pressure (AP) or relative decompression ( P i / P f ) as predictive indices of decompression stress. 4. 4. The results indicate that experimental determination of the critical conditions for bubble formation in blood and tissues in vivo may be possible with such material to a precision not possible in mammalian studies.