A histochemical method for demonstration of acid inorganic polyphosphatase (IPPase) was applied to developing and adult fish of the Great Lakes region. Enzyme distribution was compared with that of alkaline glycerophosphatase (GPase) and with IPPase distribution in the frog. In the gastrulae of the minnow Pimephales notatus, IPPase was present in the presumptive yolk sac and absent in the embryonic shield, while GPase was limited to the mesodermal layer of the embryonic shield. In the newly hatched minnow fry, IPPase was extremely abundant in the yolk sac, which had no GPase. Distribution of IPPase in other organs paralleled that found in the tadpole. Study of other fish species now in progress will test the correlation between IPPase and utilization of yolk. The mesonephric kidney of the adult stickleback, Eucalia inconstans, contains both IPPase and GPase in the proximal convoluted tubules. IPPase, however, is present in great abundance in the modified pronephric component of the kidney, which has no GPase. The study of other species is in progress.