Optical properties of the marine diatom Thalassiosira pseudonana were examined in nitrate-limited semicontinuous cultures for growth rates (μ(N)) varying from 0.22 to 1.32 d-1. Two experiments were conducted at each growth rate in which sampling of the culture was done at either the dark-to-light (D-L) or light-to-dark (L-D) transition of a 12-h photoperiod. Optical cross sections and efficiency factors for absorption, scattering, and attenuation were calculated from measurements of the spectral absorption and beam attenuation coefficients, cell concentration, and the size distribution of cells in suspension. The refractive index of cells, relative to seawater, was calculated from these data using an inverse method. Nitrogen limitation strongly influences the optical properties of this species. Absorption cross sections increased more than twofold with increasing μ(N), predominantly caused by increases in the imaginary part of the refractive index, n'. At the red absorption peak, n'(673) increased 140% and was strongly correlated with intracellular Chl a concentration. For most visible light wavelengths, scattering cross sections were not a strong function of growth rate but were consistently largest in the L-D transition experiments. The difference between D-L and L-D experiments at any given μ(N) increased with increasing growth rate; for example, scattering cross sections at 660 nm in the L-D experiments were 25% (μ(N) = 0.22 d-1) and 90% (μ(N) = 1.32 d-1) larger than in the respective D-L experiments. The real part of the refractive index at λ = 660 nm varied from 1.024 to 1.039 among all experiments and was positively correlated with intracellular carbon concentration. The Chl a-specific absorption coefficient decreased with increasing μ(N) (e.g. 43% decrease in the blue Soret band) in response to decreases in the carotenoid-to-Chl a ratio and increased pigment packaging. The Chl a-specific scattering coefficient at 660 nm decreased from 0.55 to 0.09 m2 (mg Chl a)-1 with increases in μ(N). Our results suggest that the optical properties of phytoplankton populations in nitrogen-limited surface waters may be significantly altered from those in deeper portions of the water column.