[14C]Methylamine uptake by free-living Rhizobium sp. ANU289 had Michaelis-Menten kinetics (apparent K(m) 6.6 μM). Uptake was competitively inhibited by ammonia (K(i) 0.4 μM) and was dependent on an energized membrane. Uptake by bacteria had an optimum at pH 7.0. Methylamine uptake by bacteroids from siratro root nodules was much slower than that by free-living bacteria at pH 7.0 but increased exponentially with the pH of the medium. Uptake by bacteroids did not show saturation kinetics and was insensitive to the presence of ammonia or uncouplers. These results suggest that free-living bacteria (grown under conditions where ammonia is limiting) have an active transport mechanism for the uptake of ammonium ions; this carrier is not operative in the symbiotic state, where passive diffusion of ammonia occurs. In the free-living state, the ammonium carrier is under genetic control, being repressed by growth on high concentrations of ammonia. Derepression occurs under conditions of nitrogen starvation.