The structure of dihydrorhizobitoxine isolated from Rhizobium japonicum has been determined as O-(2-amino-3-hydroxypropyl) homoserine. A new amino-acid, formerly called unknown Y, was originally isolated from nodules produced by Rhizobium japonicum on roots of soybean (Glycine max (L.) Merr.), and was subsequently isolated from solution cultures of Rhizobium japonicum with mannitol as a carbon source. It is here shown to be O-(2-amino-3-hydroxypropyl)homoserine (CH2OH-CHNH2-CH2-O-CH2CH2CHNH2CO2H) and given the trivial name dihydrorhizobitoxine to indicate that it is a structural analogue of the antimetabolite rhizobitoxine produced by the same bacterium.