In the course of our screening for new herbicidal antibiotics, we found that a soil isolate SC-1688 collected at Gonohe, Aomori Prefecture, produced an active material, homoalanosine. Homoalanosine having a herbicidal activity was isolated from the culture filtrate of a soil isolate SC-1688 which was classified as Streptomyces galilaeus. The chemical structure of homoalanosine was determined to be L-2-amino-4-nitrosohydroxyaminobutyric acid by analyses of spectral and biological data. The antibiotic has high herbicidal activity at low concentrations against especially commoncocklebur and ladysthumb among the tested weeds and crops. Foliar application of this antibiotic inhibited the growth of roots and buds, indicating that homoalanosine had a systemic herbicidal activity. Although the racemate of this antibiotic had already been chemically synthesized as a homolog of alanosine and its insecticidal activity had been also reported, its isolation from natural sources and herbicidal activity have not been reported. This paper deals with the taxonomy of the producing strain and fermentation, purification procedures, structure determination, herbicidal activity and mechanism of action of homoalanosine.