In searching for new biologically active natural products from medicinal plants, we recently isolated the anti-inflammatory constituents, entadamides A, B and C, from Entada phaseoloides Merr.1-4) Continuing our chemical investigation of crude drugs used in Thailand, we isolated a new seven-membered lactam, (+)-acacialactam (1) from the viable seeds of Acacia concinna DC. (Leguminosae). These seeds are used as a source of folk medicine for skin diseases in Thailand and the tropical countries. The constituents previously isolated from this plant are several saponins5-8) and a flavonoid.6) However, there are no reports on the constituents of the basic fraction in the seeds. This communication deals with the isolation and structure determination of 1 from the basic fraction in the seeds of A. concinna DC.