A model was developed, based on data collected in the Southern California Bight region, to assess the effect of zooplankton grazing on the attenuation of light due to suspended particles. Diel vertical distribution and grazing activity of the principal zooplankton grazers in the coastal waters of southern California were studied during mid-March, 1986. Calanus pacificus exhibited vertical migration, but Acartia spp. and Paracalanus spp. did not. All species had a diel feeding rhythm, whether or not they migrated; grazing activity, measured by the gut fluorescence method, increased at night. Model parameters are temperature, particle doubling rate, particle size-frequency distribution, zooplankton grazing efficiency and zooplankton size-frequency distribution. With parameters at their standard values, the diffuse attenuation coefficient, Kp, remains approximately constant, decreasing by only 3.5% in one 24-h cycle. The model is most sensitive to changes in temperature and, secondly, to changes in the abundance of grazers. Without grazers, and at the reference value for particle doubling rate, Kp is expected to increase by 8.2% d−1. At the upper limit of zooplankton abundance grazing produces a decrease in Kp of 63.5% d−1; at the upper limit of particle growth rate, Kp increases by ≥50% d−1. We conclude that macrozooplankton can have a major effect on the optical characteristics of sea water.