ABSTRACT. The perennially ice-covered lakes of Antarctica have hydrodynamically stable water columns with a number of vertically distinct phytoplankton populations. We examined the photosynthesis-irradiance characteristics of phytoplankton from four depths of Lake Bonney to determine their physiological condition relative to vertical gradients in irradiance and temperature. All populations studied showed evidence of extreme shade adaptation, including low I-k values (15-45 uE/m²/s) and extremely low maximal photosynthetic rates (P-Bm less than 0.3 ug C/ug chl a/h). Photosynthetic rates were controlled by temperature as well as light variations with depth. Lake Bonney has an inverted temperature profile within the trophogenic zone that increased from 0°C at the ice-water interface to 6°C from 10 to 18 m. Deeper phytoplankton (10 m and 17 m) were found to have photosynthetic capacities (P-Bm) and efficiencies three to five times higher than those at the ice-water interface. However, Q-10 values were only ca. 2 for P-Bm (no temperature dependence was evident for efficiency), suggesting that a simple temperature response cannot explain all the differences between populations. Lake Bonney phytoplankton (primarily cryptophytes and chlorophytes) had photosynthetic characteristics similar to diatoms from other physically stable environments (e.g. sea ice, benthos) and may be ecologically analogous to multiple deep chlorophyll maxima.