Injections of bovine parathyroid hormone (PTH 1-34) and homogenates of corpuscles of Stannius produce hypocalcemia in male killifish and tilapia adapted to calcium-deficient seawater or fresh water, respectively. In fish from water with normal calcium concentrations no effects are noticeable. These results suggest similarity in bioactivity between PTH, the hypercalcemic hormone of terrestrial vertebrates, and the hypocalcemic factor of the corpuscules of Stannius in teleost fish.