Cayo del Grullo is a hypersaline estuarine bay on the south Texas Gulf Coast. Most surficial sediment is mud, with sand predominating around the periphery of the bay. Influx of sediment is small and distribution of the several lithofacies and biofacies within the bay is related directly to the prevailing energy conditions. The mud comes from several sources, from Laguna Madre, from erosion of surrounding Pleistocene bluffs, and from intermittent streams during heavy rains. Organic content of bottom sediments is inversely proportional to grain size. Oolitic aragonitic sands are present in the bay and are more extensive than previously reported. Oolitic grains are precipitated and deposited in the high-energy swash zone. Other carbonate material in the sediments is primarily pelecypod shell fragments. Concentration of carbonate material is a function of currents and wave activity. A few species of Foraminifera and Ostracoda which can tolerate severe fluctuations of salinity comprise the microfauna.