Three cyclic heptapeptide hepatotoxins, [Asp3, ADMAdda5, Dhb7] microcystin RR, [Asp3, ADMAdda5, Dhb7] microcystin HtyR and [Asp3, ADMAdda5, Dhb7] microcystin LR, were isolated from a Nostoc species. Their structures were assigned on the basis of $H$ and $^{13}$ C NMR, HR FAB mass spectrometry and amino acid analysis using GC-mass spectrometry on a chiral capillary column. All three microcystins contained dehydrobutyrine (Dhb), instead of dehydroalanine (Dha), D-aspartic acid, instead of D-ervthro-Bmethylaspartic acid (D-MeAsp), and 9-acetoxy-3-amino-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid (ADMAdda), instead of the corresponding 9-methoxyl derivative (Adda). This is the first report on the identification of Dhb-containing microcystins from Nostoc.