East Bay, an arm of the Galveston Bay system of Texas, was studied during the summers of 1954, 1955, and 1956. The first study was made with the Bay in its natural state, the second investigation followed the excavation of a pass (Rollover Pass) that permitted considerable interchange between the Gulf of Mexico and East Bay, and the third study was subsequent to partial blocking of the Pass. Considerable evidence from salinity distribution and fluctuations in population density of the dominant fish and shrimp species suggests a rapid physical and biological reversion toward the natural condition, following the blocking of the Pass. In addition, the presence or absence of certain fish and shrimp forms was correlated with changes in the Bay (DBO)