Larger scale isolation of microcystins, cyclic heptapeptide hepatotoxins, from a water bloom of Microcystis spp. collected from Homer Lake (Illinois) gave the previously reported 1-5, additional quantities of [L-MeSer7]microcystin-LR (6), and microcystin-(H4)YR [8, (H4)Y = 1',2',3',4'-tetrahydrotyrosine], which were previously isolated in insufficient amounts to complete the structure assignment, and seven more microcystins, 9-15. A general method for assigning the structures of cyclic peptides containing α,β-unsaturated amino acid unit(s) developed with nodularin, a cyclic pentapeptide hepatotoxin, was applied to confirm the previously assigned structures of 1-5 and to assign the structures of [D-Asp3]microcystin-LR (9) and the new microcystin-HilR (10, Hil = homoisoleucine). The method consists of linearization of a cyclic molecule by a one-pot reaction sequence (ozonolysis followed by NaBH4 reduction) and tandem FABMS (FABMS/CID/MS) analysis of the product (linear peptide). A new microcystin, 11, was assigned the structure [L-MeLan7]microcystin-LR (MeLan = N-methyllanthionine) and synthesized from 1 and L-CYS. Four linear peptides 12-15, which are reasonable biogenetic precursors of the cyclic compounds, were also assigned structures based on their FABMS/CID/MS data.