ABSTRACT. Lakes of the dry valley region near McMurdo Sound, Antarctica, which are perennially ice-covered (approximately 4 m thick) and contain only microorganisms in the plankton, present unique systems for studies in hydrologic optics. Profiles of the flux and spectral distribution of photosynthetically available radiation (400-700 nm) and the optical properties of suspended particulate material were measured for Lake Bonney and Lake Fryxell. The spectral quality of light immediately beneath the ice was similar in both lakes and is comparable to previously published results for nearby Lake Hoare; irradiance was always less than 50 umol photons/mē/s. However, the concentration and vertical distribution of phytoplankton biomass (i.e., chlorophyll) differed greatly among the three lakes and was similar to the trend in light attenuation of Lake Fryxell >> Lake Hoare > Lake Bonney. The same relationship between chlorophyll and light attenuation was observed as chlorophyll concentrations increased over the growth season in Lake Bonney. The wavelengths of maximum transmission through the water columns were in the range 507-540 nm with longer wavelengths (>600 nm) attenuated most rapidly. Spectral downwelling irradiance was attenuated primarily by phytoplankton pigments at shorter wavelengths (<520 nm) and by water at longer wavelengths. An analysis of absorption due to water, phytoplankton, detritus, and, by difference, gilvin (dissolved materials) showed that water was the dominant absorber (38-75% of the total absorption coefficient) but that phytoplankton was usually the most important of the variable components (11-47%). Exceptions were the dominance of phytoplankton following the seasonal introduction of glacial stream water. In terms of the relative importance of light-absorbing components, the dry valley lakes appear to be most similar to oligotrophic seas and would be defined as case 1 waters.