The ability of 2 strains of T. ferrooxidans to oxidize FeSO4 and pyrite in the presence of 4 detergents was obsd. Stimulation or inhibition of the cells were dependent on the combination of substrate and detergent. With Triton X 100 the optimum addn. proved to be at 10-5/-6M, which resulted in an increase of oxidn. of 10-26%. The interaction of Brij 58, deoxycholate, and SDS with the cells was varied. Slight stimulation as well as inhibition were noted.