Halogenated aliphatic and aromatic hydrocarbons are the most important organic contaminants of shallow water-table aquifers. Such aquifers have recently been shown to harbor an indigenous microflora. Organisms in authentic and uncontaminated samples of unconsolidated subsurface material degraded styrene and toluene. Samples from one site also degraded chlorobenzene. Samples from three sites could not degrade tri- or tetrachloroethylene, 1,1,1- or 1,1,2-trichloroethane, 1,1- or 1,2-dichloroethane or chloroform at rates greater than 2% per week.