SYMBIOSIS with nitrogen-fixing bacteria (rhizobia) allows legumes to survive in nitrogen-poor soils. The nitrogen-fixing bacteroids are found inside root nodule cells within the symbiosome, an organelle bounded by the peribacteroid membrane(1). Across this membrane the plant receives fixed nitrogen in exchange for reduced carbon(2). It has been assumed that fixed nitrogen is released from the symbiosome as either NH3 or NH4+, but until now the transport mechanism was unknown(3-5). We report here the use of patch-clamp techniques to show, in membrane patches on isolated symbiosomes, smoothly activating currents that are passive and equivalent to the influx of cations to the plant cytoplasm, The currents are largest with NH4+ as the cation at physiological concentration and they are blocked by calcium ions on the bacteroid side of the membrane. The characteristics of the currents are more like those of a channel than of carrier-mediated transport, III vivo nitrogen would move passively as NH4+ to the cytoplasm upon energization of the membrane and calcium ions may be involved in regulation of the flux.