Benzenehexachloride (BHC) and its gamma isomer were toxic against bacteria that oxidize ammonia to nitrite, and those that oxidize nitrite to nitrate, in solution media inoculated with soils. They were not toxic against nitrifying bacteria in soils nor against those of a vegetable compost in solution cultures. They were also toxic against bacteria that oxidize thiosulphate in solution cultures of mineral soils. Additional organic matter reduced toxicity of the gamma isomer. The toxic component was in the benzenehexachloride and not in the "filler" materials. The gamma isomer was less effective than some other component or combination of components of the BHC. Neither the BHC nor the gamma isomer stimulated nitrification.