41a-homoyessotoxin (1), a new analogue of yessotoxin (7), was isolated from extracts of Protoceratium reticulatum grown in culture. In addition, 9-methyl-41a-homoyessotoxin (2) and a nor-ring A-yessotoxin (3) were identified as the major components of a mixed fraction. The structural information was initially obtained from LC-UV and LC-MS3 chromatograms, with subsequent definitive structure determination by NMR, LC-MS/MS, and high-resolution MS. Full 1H and indirectly detected 13C NMR assignments for all but two carbon atoms were obtained from ca. 100 microg of 1. Full 1H NMR assignments for 2 and 3 and identification of three new heptanor-41-oxo-analogues of 3 (4-6) during LC-MS3 analysis of a fraction containing 1-3 and 7 are also reported.