Two oxalyl derivatives of a,y-diaminobutyric acid have been isolated from the seeds of Lathyrus latifolius. The reversible transfer of the oxalyl group between the a- and y-amino groups of the amino acid with the establishment of an equilibrium has been demonstrated. a-Oxalylamino-β-aminopropionic acid, an isomer of the neurotoxin α-amino-β-oxalylaminopropionic acid, has been found together with the neurotoxin in seed extracts of this same species and also in seed extracts of L. sativus, L. clymenum and L. cicera, species implicated in classical "lathyrism", a neurolathyrism, which affects man and domestic animals. An analogous transfer of the oxalyl group between the α- and β-amine groups of α,β-diaminopropionic acid has also been shown. The phylogenetic and toxicological significance of these findings is discussed.