The spectral variations of the efficiency factors for absorption, total scattering and backscattering have been computed via Mie theory using a three-layered sphere model, with the size distribution function and the spectral values of the refractive index of each layer as input parameters. When compared with the results of a model for homogeneous spherical cells (with an equivalent bulk refractive index), these theoretical predictions allow the modifications of the efficiency factors due to heterogeneities within algal cells to be assessed. Such a comparison has been performed for a coccolithophorid suspension (Emiliania huxleyi), for which the spectral values of the refractive index have been derived from the experimental absorption and scattering coefficients. While the internal structures induce insignificant modifications in absorption and only weak modifications in total scattering, they appear to be able to increase the backscattering efficiency by a factor as high as 50, depending mainly on the calcite shell thickness. The internal structures also induce spectral changes in backscattering.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.