Biotic and abiotic degradation of di-n-butylphthalate (DBP) in water and sediment/water systems from six different sites was investigated under laboratory conditions. Water and underlying sediment were collected from freshwater and estuarine sites in Florida, Mississippi, and Louisiana, amended with DBP, and incubated under conditions of constant temperature and pH. Formalin-sterilized systems from each site were included to evaluate abiotic degradation. DBP disappearance was rapid in all microbially active systems and substantially reduced in the presence of formalin. In biologically-active systems actual time to half the starting concentration ranged from 1.0 to 4.8 days in sediment/water mixtures and from 1.7 to 13.1 days in water alone. Adaptation of microbial populations to degrade DBP was indicated in six of the nine evaluations conducted. The presence of sediment significantly increased biodegradation rates in five of the six sites.