Enterobacter amnigenus GG0461, isolated and identified from greenhouse soils, has a high activity of nitrate uptake in the Pseudomonas agar factor medium. Although nitrate is not essential for the growth of this microorganism, it increases the growth rate. To characterize the microbial nitrate uptake, we measured the effect of chlorate on the uptake, since chlorate is an inhibitor of plant nitrate uptake. In the absence of nitrate, the growth of E. amnigenus was inhibited by chlorate at concentrations higher than 1 mM. In the presence of 50 mM nitrate, nitrate uptake was completely inhibited by 30 mM of chlorate, and growth was suppressed by chlorate at concentrations above 10 mM. In the presence of chlorate, the growth and nitrate uptake gradually improved when nitrate concentration increased. To overcome the inhibitory effect of chlorate, the nitrate concentration was required to be 2.5 times higher than that of chlorate, suggesting that there exists competition between chlorate and nitrate for the binding site of the nitrate transporter. Chlorate-induced inhibitions of bacterial nitrate uptake and growth seemed to have different mechanisms: a competitive inhibition in the former case and the formation of chlorite, a toxic reduced metabolite of chlorate, in the latter case.