Recently, freshwaters in Thailand have been eutrophicated by industrial andmunicipal wastewater. Toxic cyanobacterial waterblooms have occurredfrequently in the freshwaters. The major toxin in the waterblooms has beenidentified as microcystin-LR and -RR and their 6(Z) forms. Also othermicrocystin variants such as microcystin-YR, -LA, -ThtyrR and -AR weredetected as minor toxins. During investigation of microcystin incyanobacterial waterblooms in Thailand, we found a novel microcystinvariant. The waterbloom sample was collected from a pond in Lumpoon, andwas consist of Microcystis aeruginosa, Microcystin was extracted withmethanol, and was fractionated using Sep-Pak ODS cartridges. Each ofmicrocystin variants were separated by HPLC using a reverse-phase column,and purified by silicagel TLC using chloroform / methanol / water (60/40/1,v/v) as the solvent. The chemical structures of the variants were analyzedusing two-dimensional NMR and HRFABMS. The waterbloom containedmicrocystin-RR, -YR, -LR and -WR as known variants, and a hydrophobicunknown variant. From the results of the analyses and determination ofamino acid configuration, the unknown microcystin variant was identified asmicrocystin-AW.