1) Nitrapyrin was less inhibitory to Nitrosomonas europaea in soil culture than in liquid batch culture.2) In the course of liquid batch culture studies, 3 strains of N. europaea were isolated which exhibited differing responses to nitrapyrin. All 3 strains however, were inhibited by 0.5 ppm nitrapyrin and stationary phase cells were found to be less sensitive than exponentially growing cells. 3) 6-chloropicoline, a hydrolysis product of nitrapyrin, was found to inhibit exponentially growing but not stationary phase cells. 4) Cells of N. europaea adhered to glass slides and during the initial stages of colonisation cells were not protected from the inhibitory effect of nitrapyrin. Once cells had become established at the solid/liquid interface there was a significant reduction in the specific growth rate compared with freely suspended cells and 0.5 ppm nitrapyrin no longer inhibited growth. 5) The specific growth rate of cells detached from the surface of a glass slide was not significantly different from that of cells adhered to the surface but significantly less than the specific growth rate of freely suspended cells. Detached cells were not significantly inhibited by 0.5 ppm nitrapyrin. 6) In the presence of montmorillonite, vermiculite and ammonia-treated vermiculite (ATV), growth of N. europaea occurred in two distinct growth phases. All three clay types reduced the inhibitory effect of nitrapyrin and inhibition was only observed during the second growth phase. In the presence of a fourth clay type, illite, growth occurred in one phase only and there was no significant reduction in inhibition by nitrapyrin. 7) Addition of Cu2+ and A13+ enhanced inhibition of N. europaea by nitrapyrin in liquid batch culture. 8) Growth of N. europaea at sub-optimal rates in chemostat culture reduced the sensitivity of cells to nitrapyrin. Productivity increased with increasing nitrapyrin concentrations and ammonia oxidizing activity per cell increased following addition of 0.5 ppm and 1.5 ppm nitrapyrin to steady state cultures. 9) Therefore the clay content of soils and the physiological state of N. europaea may reduce the effectiveness of nitrapyrin when applied in the field.