N2-fixation by Rhizobium-legume symbionts is of major ecological and agricultural importance, responsible for producing a substantial fraction of the biosphere's nitrogen. On the basis of 15N-labelling studies, it had been generally accepted that ammonium is the sole secretion product of N2-fixation by the bacteroid and that the plant is responsible for assimilating it into amino acids. However, this paradigm has been challenged in a recent 15N-labelling study showing that soybean bacteroids only secrete alanine. Hitherto, nitrogen secretion has only been assessed from in vitro 15N-labelling studies of isolated bacteroids. We show that both ammonium and alanine are secreted by pea bacteroids. The in vitro partitioning between them will depend on whether the system is open or closed, as well as the ammonium concentration and bacteroid density. To overcome these limitations we identified and mutated the gene for alanine dehydrogenase (aldA) and demonstrate that AldA is the primary route for alanine synthesis in isolated bacteroids. Bacteroids of the aldA mutant fix nitrogen but only secrete ammonium at a significant rate, resulting in lower total nitrogen secretion. Peas inoculated with the aldA mutant are green and healthy, demonstrating that ammonium secretion by bacteroids can provide sufficient nitrogen for plant growth. However, plants inoculated with the mutant are reduced in biomass compared with those inoculated with the wild type. The labelling and plant growth studies suggest that alanine synthesis and secretion contributes to the efficiency of N2-fixation and therefore biomass accumulation.