This report concerns physiological function of mycosporinelike amino acids (MAA) as an active defense against the photooxidative effects of sunlight in marine organisms. Mycosporine glycine (MG) is a representative member of MAA family and was found to effectively suppress various detrimental effects of the Type-II photosensitization in biological systems, such as inactivation of mitochondrial electron transport, lipid peroxidation of microsomes, hemolysis of erythrocytes and growth inhibition of Escherichia coli. The presence of MG in solutions of eosin Y or methylene blue resulted in a marked decrease in the level of singlet oxygen (¹O₂) produced by the sensitizers under illumination. The rate constant of ¹O₂ quenching by MG was determined to be 5.6 × 10⁷ M⁻¹s⁻¹ by the time-resolved ¹O₂ luminescence decay method, which is higher than, or at least comparable to, the values for ¹O₂ reaction of well-known quenchers such as 1,4-diazabicyclo[2,2,2]octane and furfuryl alcohol. The results suggest that MG probably together with some other active MAA may play an important role in protecting marine organisms against sunlight damage by eliminating ¹O₂ generated from certain endogenous photosensitizers.