Hone tissue simultaneously provides for biomechanical and homeostatic re quirements. Current concepts of the. regulation of calcium and phosphorus ions in higher vertebrates stress the integrated role of the parathyroid gland, vitamin I) and the vital osteocytc. The phylogenetic history of this regulatory mechanism makes it clear that the tetrapods were the earliest forms to acquire both a parathyroid gland and a consistently cellular bony skeleton. Kish, on the other hand, are characterized by a total lack of a parathyroid gland (or homologue) and by an osseous histology which is either cellular or acellular (ie, either with or without enclosed ostcocytes). Marine fish live in a calcium rich environment while fresh-water species may exist in waters which are relatively impoverished with respect to this ion. It is, therefore, theoretically possible that Iresh-water fish may obtain a selective ad vantage in possessing some mechanism for mobilization of internal calcium (and phosphorus) stores in the face of environmental deprivation. It has been established that enclosed osteocytes are the probable target for Immorally mediated homeostatic controls. Further, ostcocytes themselves have been shown to he capable of initiating osseous resorption. Therefore, it is reasonable to speculate, as some have, that cellular honed fish should he more successful in fresh-water adaptation than acellular species. Some of the morphological, biochemical, biophysical and physiological charac teristics of fish bones have been investigated in this laboratory (Moss [1961a, b. c; 1962a. h: 1963a. b, c]: Moss and I'osnrr [1961], Moss and Freilich [1963]). The majority of the fish reported on the former studies were marine (cf. Moss [1961b]). Accordingly, a histological survey of fresh w ater fishes was made. The present data do notsupport the hypothesis that fresh-water fish are predominantly cellular. With the consequent elimination of skeleton histology as a basis lor environmental adaptation, we are now freer to consider other mechanisms by which piscine hone may participate to mineral homeostasis.