The mysid shrimp Neomysis mercedis is a major prey of striped bass Morone saxatilis in the inland delta and estuary of the Sacramento and San Joaquin rivers, California. Its abundance during 1968–1981 was highest between 1.2 and 4.6‰ surface salinity. Cross-delta flow of water to large pumping plants and shallow river channels with high current velocities limited the upstream extent of the shrimp. The population shifted spatially in response to salinity changes caused by variations in river outflow. Annual July to October abundance indexes were highest from 1968 to 1975, and lowest during the drought years 1976 and 1977. Regression analysis showed that population size was negatively related to salinity intrusion and positively related to the abundance of the copepod Eurytemora affinis, an important food item of the shrimp. High N. mercedis populations appear dependent on adequate food supply and minimal salinity intrusion into the western delta.