Nitrosomonas europaea is capable of incorporating exogenously supplied amino acids. Studies in whole cells revealed that at least eight amino acids are actively accumulated, probably by the action of three different transport systems, each with high affinity (mu-molar range) for several amino acids. Evidence for the action of secondary mechanisms of transport was obtained from efflux, counterflow and exchange experiments. More detailed information was obtained from studies in liposomes in which solubilized integral membrane proteins of N. europaea were incorporated. Uptake Of L-alanine in these liposomes could be driven by artificially imposed pH gradients and electrical potentials, but not by chemical sodium-ion gradients. These observations indicate that L-alanine is transported by a H+/alanine symport system. The ecological significance of secondary amino acid transport systems in autotrophic ammonium-oxidizing bacteria is discussed.