Forty-six samples of apatite from lower vertebrates (ostracoderms, placoderms, elasmobranchs, actinopterygians, and crossopterygians), ranging in age from Silurian to Recent, were analyzed to determine each specimen's 87Sr/86Sr isotopic ratio and content of the elements Na, F, Sr, and La. Our aim was to determine whether these chemical parameters can yield reliable information about the paleosalinity conditions of the ancient fish habitats. In recent times, the uniform 87Sr/86Sr isotopic ratio and Na, F, and Sr abundances of sea water in general differ characteristically from the corresponding ratios and abundances of fresh waters and low-saline brackish waters. For evaluation of the data on fossils, analyses were made on recent skeletal-fish apatite derived from different waters with known Sr-isotopic ratio and salinity. Salinity interpretations based on our chemical analyses are generally congruent with the salinity conditions indicated by other paleoenvironmental parameters inherent in the fossil-bearing strata. Some fossil specimens, however, gave signals seemingly incompatible with prevailing opinions regarding the environment in which the embedding strata formed. In some cases, the divergences could be accounted for by erratic occurrences of reworked material. Other cases, such as ostracoderm and placoderm remains from the Old Red Sandstone, are less easy to unravel. The method tested in this study may be a useful tool, clarifying, for example, salinity conditions during formation of various Old Red Sandstone deposits.