The two-population absorption model of Sathyendranath et al. is extended to retrieve the specific absorption coefficients (absorption per unit concentration of chlorophyll-a) of the component populations of phytoplankton. The model relates the absorption coefficient of phytoplankton to chlorophyll-a concentration, assuming that the assemblages of phytoplankton comprise mixtures of two populations whose proportions vary as the total concentration of cells changes. The model is applied to in situ data collected from six regions during 34 cruises. The model compares well with earlier models of phytoplankton absorption but brings the additional advantage of parameters that have clear bio-optical and biological interpretation. Size structure of the phytoplankton populations was inferred from the values of the specific absorption coefficients. The results are consistent with pigment analyses performed on the same samples. Seasonal analysis of data from the northwest Atlantic, southeast Pacific, and the Arabian Sea showed significant changes in the spectral form and magnitude of the specific absorption coefficients of small- and large-celled populations, which appear to be related to changes in species composition. The model serves thus as an optical tool to explore the large-scale biogeography of phytoplankton.