Abstract 1. 1. Young goldfish, Carassius auratus, were fed a calcium-deficient diet for 8 or 56 days and then their net uptake of calcium from the aquatic environment was examined using 45Ca. 2. 2. They showed normal growth and the normal level of plasma calcium in both experiments. 3. 3. In the short-term experiment, no dietary effect was found in the uptake of environmental calcium. In the long-term experiment, however, the rate of uptake was significantly stimulated by 18%. 4. 4. The results indicate the presence of a compensatory action in calcium uptake between dietary and aquatic sources.