Abstract 1. The lower incipient lethal salinity for non-moulting Palaemon ritteri is 10% at 20°C. 2. The upper incipient lethal salinity for non-moulting P. ritteri is 47·5% at 20°C. 3. Recently moulted P. ritteri are unable to tolerate salinities of 15, 20 or 40%. 4. Present salinities in some coastal environments of the northern Gulf of California are close to the upper lethal salinity for moulting P. ritteri. 5. Brine effluents from a proposed nuclear desalting plant may cause local salinity conditions in excess of the upper incipient lethal salinity for P. ritteri.