The in vitro photoinduced reactions of the mycosporine-like amino acids (MAA) usujirene and palythene were studied by monochromatic stationary irradiation at 366 nm. High-performance liquid chromatography analysis of the irradiated aqueous solution of usujirene indicated a low photoreactivity on the basis of the observed photodecomposition quantum yield of phi(-U) = (2.86 +/- 0.80) x 10(-5), which can be partially accounted for by the cis-trans photoisomerization of usujirene to palythene (phi(U-->P) = [1.71 +/- 0.13] x 10(-5)). However, palythene in aqueous solution showed a higher photostability than did usujirene under equivalent conditions, establishing a photostationary mixture of cis-trans isomers with a relative composition of palythene-usujirene (11:1). These results may explain the preferential in vivo accumulation of palythene relative to that of usujirene observed in several dinoflagellate species.