Biological degradation of oil-based drilling muds and production pit sludges are examined using batch reactors. Microbial populations from five different cultures were screened as to their capability of survival in degradation of the oils present. Two microorganism cultures, one developed from raw domestic wastewater and the other from a refinery biological treatment system, demonstrated superior acclimation degradative properties based on substrate utilization and oxygen uptake rates. No toxic or inhibitory properties of the wastes were observed. Biological treatment has the potential to satisfactorily biodegrade wastes for disposal according to recently established Louisiana requirements, and also as a preconditioning step prior to conventional dewatering techniques.