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Marine phytoplankton are sensitive to inhibition of photosynthesis by solar ultraviolet (UV) radiation, although sensitivity varies, depending on the growth environment. A mechanism suggested to increase resistance to UV inhibition is the accumulation of UV‐absorbing compounds, such as the mycosporine‐like amino acids (MAAs) found in many marine organisms. However, the effectiveness of these compounds as direct optical screens in microorganisms has remained unclear. The red‐tide dinoflagellate Gymnodinium sanguineum Hirasaka accumulates about 14‐fold more MAAs (per unit of chlorophyll) in high (76 W·m⁻²) than in low (15 W·m⁻²) growth irradiance. Biological weighting functions were estimated for UV inhibition of photosynthesis and showed that the high‐light‐grown cultures have lower sensitivity to UV radiation at wavelengths strongly absorbed by the MAAs. The time course of photosynthesis …