ABSTRACT: Abstract: Diel variation in chlorophyll a-specific absorption coefficients, a*_ph (λ), m² (mg chl a)^–1, was examined to study the effect of ultraviolet radiation (UVR) on a*_ph (λ) of the prymnesiophycean Isochrysis galbana (Parke). A continuous culture was exposed for 2 wk to a 12 h light:12 h dark cycle of photosynthetically available radiation (PAR) supplemented with UVR. Photosynthetic and photoprotective pigments as well as cell density and diameter were also determined every 2 h for 2 d to confirm the periodicity of a*_ph (λ). A distinct diel variation was observed, with maxima toward the end of light periods and minima toward the end of dark periods. The magnitude of diel variation in a*_ph (440) and a*_ph (676) was 14 and 6.6%, respectively. To reconstruct the absorption coefficient, the absorption efficiency factors Q_a(λ), were determined using intracellular chl a concentration, cell diameter, and chl a-specific absorption coefficients after solubilization on Triton X, a*_TX (λ). The main source of diel variation for Q_a (λ) appears to be a*_TX (λ). Diel variation in the absorption ratio of photoprotective pigments to chl a was observed to be more significant than that for the absorption ratio of other photosynthetic pigments to chl a. The diel variations in a*_ph (λ) were primarily caused by changes in pigmentation of photoprotective pigments due to UVR.