A method was developed for selecting hybrid trout for deep-swimming ability, for use in a breeding program to combine in one strain the early-maturing character of brook trout (Salvelinus fontinalis) with the deep-swimming ability of lake trout (S.namaycush). The method involves testing hybrids in pressure tanks and selecting individuals that, like lake trout, retain most of their swimbladder gas during the test period. For a sample of F2hybrids the range of pressures at which the fish floated when anaesthetized was almost entirely between the medians for samples of the two parent species. Successive tests of marked individuals showed good repeatability of flotation measurements. The method is concluded to be reliable for large-scale selection of fish with ability to retain swimbladder gas.