When a freshwater fish, Tilapia mossambica (now renamed Sarotherodon mossambicus) is exposed to a salinity stress, extensive changes are noted in the properties of the mitochondria isolated from the gill tissue. Our efforts were directed toward ascertaining the response of the gill mitochondria to an ion-osmotic shock to which the organism is exposed. Notable among them are the loss of ability to phosphorylate externally added ADP, decreased 45Ca++ uptake, lower transhydrogenase levels, and nonresponse to ATP + Mg++ for contraction. During this period there is a large influx of calcium into the mitochondria. Continued exposure to the stress situation reverses virtually all the changes to the freshwater levels.