Inhibition of growth of axenic cultures of Nitrosomonas europaea by nitrapyrin was investigated in liquid culture and in soil. In liquid culture, exponentially growing cells were more sensitive than stationary-phase cells, possibly due to a requirement for uptake of nitrapyrin, metabolism of nitrapyrin, or both before inhibition. Differences in sensitivity were observed between the parent strain and two strains, sp1 and sp2, that were selected through repeated subculturing. These differences were reflected in the length of the lag period induced by nitrapyrin and in the specific growth rate and were due to different bactericidal and bacteriostatic effects. Soil provided significant protection from inhibition, with concentrations of nitrapyrin approximately one order of magnitude greater than those required for equivalent inhibition in liquid culture. The data show that strain differences alone do not explain differences in sensitivity between nitrification in soil and in liquid culture and suggest that the inhibitor may be more effective against actively nitrifying soils.