A 2-amino-2-butenoic acid(Dhb)-containing microcystin was isolated from freshwater eyanobacteria Oscillatoria agardhii. By the extensive NMR analyses, the structure of the new microcystin was identified as [D-Asp3,Dhb7] microcystin-RR(1).Some strains of Oscillatoria agardhii as water bloom-forming cyanobacteria, produce cyclic heptapeptide hepatotoxins, named microcystins1. The general structure of the microcystins is cyclo(-D-Ala-X-D-MeAsp-Z-Adda-D-Glu-Mdha-), where X and Z are variable t-amino acids, D-MeAsp is D-erythro-B-methylaspartic acid, Mdha is N-methyldehydroalanine, and Adda is (2S, 3S, 8S, 9S)-3-amino-9-methoxy-2,6,8-trimethyl-10phenyldeca-4,6-dienoic acid. The two acidic amino acids, D-MeAsp and D-Glu, are connected by an isolinkage. During investigations of cyclic peptide toxins of O. agardhii, we found a 2-amino-2-butenoic acid(dehydrobutyrine, Dhb)-containing microcystin, [D-Asp3,Dhb7]microcystin-RR(DHBmicrocystin-RR, 1). In HPLC analysis, the retention time of DHBmicrocystin-RR was the same as that of [Asp3]microcystin-RR(desmethylmicrocystin-RR)1. Desmethylmicrocystin-RR was isolated from a strain of O. agardhii (NIES-595) collected from a lake in Ireland, the structure was identified by 1H- and 13C- NMR and HRFABMS analyses. The UV absorption spectra and the intensities of DHBmicrocystin-RR and desmethylmicrocystin-RR were also same. Furthermore, in the HRFABMS analysis, the molecular formula of DHBmicrocystin-RR agreed well with that of desmethylmicrocystin-RR. However, the 1H-NMR spectrum of DHBmicrocystin-RR was clearly different from that of desmethylmicrocystin-RR. We now describe the isolation and structure elucidation of 1.