Freshwater physostomous fish, possessing open swim bladders (dace, sucker, chub, bullhead, trout), and physoclistous fish, having closed swim bladders (perch, bass, pumpkin-seed, stickleback), were sealed in bottles of water saturated with air and containing various amounts of dissolved carbon dioxide. After asphyxiation, an analysis of the swim bladder gases showed that the swim bladder had gained some carbon dioxide and lost some oxygen. Fish with closed swim bladders possessing retia mirabilia and ovals, vascular networks that facilitate gas exchange, show a markedly greater exchange of gases during the experiments than fish with open swim bladders which have no intensely vascular regions on the swim bladder wall. The oxygen that leaves the swim bladder during asphyxiation is probably of no significant value to the fish for respiration.