When microcystin-LR was exposed to UV, three major nontoxic compounds were formed. These compounds were identified as [4(E),6(Z)-Adda5]- and [4(Z),6(E)-Adda5]microcystin-LR, which are geometrical isomers of the Adda [3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4(E),6(E)-decadienoic acid] moiety of microcystin-LR, and a novel compound, tricyclo-Adda [(2S,3S,1'R,3'S,4'S,5'R,6'R,7'R)-3-amino-5-(4',6'-dimethyl-3'-methoxy tricyclo[18.104.22.168(1',5')]undeca-8',10'-dien-6'-yl)-2-methy l-4(E)-pentenoic acid]-containing microcystin-LR ([tricyclo-Adda5]microcystin-LR), which was formed by [2 + 2] addition between the benzene ring and the double bond at position 6-7 of the Adda moiety of the microcystin. The geometrical isomers were formed reversibly, and their equilibrium constants were almost the same. [Tricyclo-Adda5]microcystin-LR was also formed reversibly and was decomposed under UV light. These results suggest that the breakdown of microcystin-LR by UV irradiation proceeds via [tricyclo-Adda5]microcystin-LR.