The effects of barite (BaSO4), a major constituent of drilling muds used in marine oil drilling operations, on meiofauna were studied in an experimental flow-through system. Aquaria containing sand or varying proportions of barite and sand received a continuous supply of unfiltered seawater from Santa Rosa Sound, Florida, for ten weeks. At the end of this period the composition, biomass and vertical distribution of the meiofauna were determined. Rotifera, Foraminifera, Hydrozoa, Turbellaria, Ostracoda, Polychaeta and Bivalvia did not exhibit significant differences between control and experimental aquaria. A marked decrease in meiofaunal density was evident in aquaria containing 5.5 cm of sand covered with a 0.5 cm layer of barite.