ABSTRACT: Salinity is a key control on species distribution in estuaries, but interspecific interactions can shift distributions of estuarine species away from physiologically optimal salinities. The distribution of the introduced calanoid copepod Pseudodiaptomus forbesi in the upper San Francisco Estuary (SFE) shifted from brackish to fresh water in 1993 following the introductions of 2 brackish-water copepods, the small but numerically dominant Limnoithona tetraspina (Cyclopoida) and the predatory Acartiella sinensis (Calanoida). The nearly simultaneous timing of these introductions complicated interpretation of the temporal change in distribution of P. forbesi. Although P. forbesi is now uncommon at salinity >~2, which might be interpreted as the result of salinity stress, short-term experiments showed high survival of adults up to salinity ~8 and of nauplii to salinity of at least 14, and reproduction was highest at salinity 5. Feeding experiments showed some overlap in diets of P. forbesi and L. tetraspina, but P. forbesi consumed a broader range of prey than L. tetraspina. Furthermore, feeding rates of the L. tetraspina population appeared insufficient to reduce prey availability to P. forbesi. Previous reports of high consumption of nauplii by A. sinensis and the clam Potamocorbula amurensis suggest that these interspecific interactions are important in constraining the distribution of P. forbesi in the upper SFE. Thus, we interpret the temporal shift in distribution of P. forbesi as due mainly to the introduction of the predatory copepod, whose high abundance may have been facilitated by the availability of a common alternative prey, L. tetraspina.