Brine shrimp, Artemia sp, (Crustacea: Anostraca), are hypo-hyperosmotic regulators able to maintain haemolymph ion concentrations within narrow limits over an external salinity range of 0.26% NaCl to supersaturated brines, The aim of the prc!sent study was to examine the effects of acid exposure during development of Artemia franciscana in seawater. The effects of exposure to pi-Is ranging from 4.5 to 8.2 on hatching success, survival throughout development, and on regulation of whole body [Na], [Cl], [K], [Mg] and [Ca] were determined for both adults and nauplii, Exposure to the lower pHs caused a significant decrease in both hatching success and survival but tolerance to acid exposure increased during development, In both adults and nauplii, mortality was correlated with increases in internal ion concentrations, The results of the present study are compared with those reported for fresh water crustaceans, and their implications for animals in euryhaline habitats are discussed.