Drilling mud, as used in exploratory drilling for oil offshore, affected the composition of estuarine communities that developed from planktonic larvae in aquaria containing sand and flowing estuarine water. Aquaria contained: sand only; a mixture (by volume) of 1 part mud and 10 parts sand; 1 part mud and 5 parts sand; or sand covered by 0.2 cm mud. For all environments, annelids, mollusks, arthropods, and coelenterates were the numerically dominant phyla collected in a 1 mm mesh sieve after eight weeks exposure; a total of 1,025 animals, representing 45 species, was collected. Annelids and coelenterates were significantly fewer in aquaria containing drilling mud than in the control aquaria. Arthropods were significantly affected only by mud cover over sand; mollusks also were diminished in this environment, but not significantly. Exposure to concentrations of drilling mud reduced not only the number of individuals, but also the frequency of occurrence of macrobenthic species.