We estimated actual 2-week mortalities of postlarval and juvenile brown shrimp in a Galveston Bay salt marsh by comparing densities of cohorts throughout the spring. Mortalities ranged between 33% and 61% in 1982 and 23% and 39% in 1987. Brown shrimp mortality in predator-exclusion cages during 1987 was less than 3%. These data and published information on food requirements, diseases, and physical tolerances suggest that predation is usually the major direct cause of brown shrimp mortality in estuarine nurseries of the northern Gulf of Mexico. Southern flounderParalichthvs lethostiginawas the dominant fish predator on brown shrimp during the spring, and appeared to be responsible for a large portion of brown shrimp mortality. In laboratory experiments, the presence of smooth corcigrassSpartina alterniflorareduced predation rates of southern flounder and some of the other fish predators examined. Predation rates in general increased in proportion to increased prey densities. Thus, low water levels in the marsh, which reduce access by brown shrimp to intertidal vegetation and increase their densities on nonvegetated bottom, probably result in increased brown shrimp mortality. Mortality and growth may also interact, and predation pressure should be reduced as brown shrimp grow and exceed optimal prey size. Indeed, mortality in the marsh appeared to decline as brown shrimp size increased.