The respiratory activity of Thiobacillus thiooxidans, even when the microorganism was incubated at pH 2.0, was not changed. In the presence of 0.1 mm 2,4-dinitrophenol (DNP), pentachlorophenol (PCP), tetraphenyl borate (TPB) and tributylamine (TBA), the respiratory activity was inhibited in acidic environments by DNP, PCP and TPB, while TBA inhibited the activity at neutral pH values. The inhibition degree by the uncouplers seemed parallel to their amount in undissociated form. The cells, when incubated at pH 2.0 or 4.0 in the presence of 0.1 mm DNP, rapid or slow H+ influx into cells was observed, respectively. The uncouplers in undissociated form may have penetrated into the membrane and caused a conformational change of the membrane, which resulted in an inactivation of the respiratory system and in an H+ influx into cells.