Barite (BaSO4), the primary component of oil drilling muds, affected the composition of estuarine communities developed from planktonic larvae in aquaria containing sand and flowing estuarine water. Aquaria contained: sand only; a mixture (by volume) of 1 part barite and 10 parts sand; 1 part barite and 3 parts sand; or sand covered by 0.5 cm of barite. For all environments, annelids and mollusks were the numerically dominant phyla collected in a 1-mm-mesh sieve after 10 weeks exposure; a total of 3020 animals, representing 59 species was collected. Significantly fewer animals and species developed in aquaria sand covered by barite than in aquaria unexposed or exposed to 1 barite:10 sand. Number of animals in aquaria containing 1 barite:3 sand also differed from that in control aquaria. Annelids were particularly affected and significantly fewer were found found in all treatments than in the control. Mollusks decreased markedly in number only in barite-covered aquaria. Barite, however, did not impede growth (as height) of the abundant clam, Laevicardium mortoni, or decrease abundance of six other phyla. Our data indicate that large quantities of this compound, as discharged in offshore oil drilling, possibly could adversely affect the colonizaton of benthic animals.