The osseous tissues of modern bony fishes are usually classified as cellular or acellular bone. The latter lacks osteocytes; the former is restricted to individual families within the Clupeiformes and Scopeliformes. Many fishes can take up calcium actively from their environment and apparently can store considerable amounts of calcium in sites other than bone. The question asto the degree that fishes can or do utilize a closed-cycle, bone—body fluid continum is a subject of controversy; those fishes that have abandoned apatites must operate as an open-cycle system. Data from several laboratories have shown that hypophysectomy does affect calcium homeostasis, at least at the serum and kidney levels. There is no evidence that the observed alterations involve metabolism of bone. The idea that the ultimobranchial body has a parathyroid-like function is an attractive one, but the evidence in favor of this view is based entirely on histological grounds and is far from decisive. Studies using extracts of this gland, and purified mammalian parathyroid hormone, are needed.