Optical oceanography models of attenuation and scattering properties often contain simple spectral relationships. Electromagnetic theory, however, predicts fluctuations in the spectra of the attenuation coefficients and scattering properties of substances at wavelengths near an absorption peak. We have modeled these effects for phytoplankton using homogeneous, two-layered, and three-layered sphere models of cell structure and using a wide range of plausible particle size distributions. The magnitude of the scattering in backward directions is affected the most. The effect on the beam attenuation spectra is relatively small compared with the effect on the absorption and scattering coefficients. The backscattering coefficient shows large variability, varying by almost a factor of 3 for some models. The results suggest that beam attenuation at any wavelength in the red shorter than the wavelength of the chlorophyll absorption peak will be insensitive to the chlorophyll content of the particles. Increases in the pigment content per volume of phytoplankton will increase the index of refraction in the infrared and therefore increase the attenuation and scattering coefficients there. ¿ American Geophysical Union 1995