Anaerobic microbial transformations of chloroacetanilide herbicides (alachlor, metolachlor, and propachlor) and aniline were investigated. Modest microbial transformations (in most cases 30-60% losses) of chloroacetanilide herbicides were found in agricultural soils incubated under anaerobic conditions for 30 days. Greater anaerobic losses were observed in anaerobic digestor sludge or rumen fluid. Anilines were not substantially transformed in any system within 30 days. The highest anaerobic activities metabolizing alachlor or propachlor occurred in anaerobic sludge. However, this biological activity could only be transferred if autoclaved sludge digestor material was added to the transfer medium, which suggests that the activities involved co-metabolism of the substrates.