The effects of drilling mud, used in oil drilling operations, on development of estuarine macrobenthic communities from settling planktonic larvae were assessed by comparing numbers and species of animals that grew in uncontaminated and contaminated aquaria for 8 weeks. Aquaria contained sand and were continuously supplied unfiltered seawater. Seven lignosulfonate-type drilling muds obtained from an active exploratory platform in estuarine waters were tested consecutively at nominal concentrations of 0.5, 5, and 50 parts per million (ppm) in the water column. Numbers of tunicates, mollusks, and annelids per aquarium were significantly (alpha = 0.05) decreased from control numbers in 50 ppm. Structural differences in communities exposed to 50 ppm from those in the control and lower concentrations were indicated by a decrease in Spearman's measure of rank correlation of species abundance and an increase in the Shannon-Weaver index of species diversity. A total of 13 species occurred in 50 ppm compared to 23 species in each of the other situations. Growth in diameter of Molgula manhattensis was significantly affected in all concentrations of mud.