The relative toxicity of seven major groundwater pollutants (benzene, chlorobenzene, propylbenzene, ethylbenzene, trichloroethylene, toluene, and styrene) and their metabolites to a soil mycobacterium (Mycobacterium vaccae strain JOB-5) that can catabolize all of these pollutants was determined. The metabolites of chlorobenzene, styrene and trichloroethylene degradation (4-chlorophenol, styrene oxide, and 2,2,2-trichloroethanol, respectively) were less toxic to M. vaccae than was their parent compound. The pollutants propylbenzene, ethylbenzene and benzene were less toxic than their metabolites (4-propylphenol, 4-ethylphenol, and phenol). Metabolites were also examined for their ability to interfere with the biodegradation of selected groundwater pollutants. The metabolites of ethylbenzene, propylbenzene and chlorobenzene biotransformation by M. vaccae were found to adversely affect biodegradation by M. vaccae. Toluene degradation by M. vaccae was inhibited by 4-chlorophenol, 4-ethylphenol and 4-propylphenol at 0.2 mM, 0.4 mM, and 0.4 mM, respectively.