The underwater light climate in Loch Ness is described in terms of mixing depth (Zm) and depth of the euphoric zone (Zeu). During periods of complete mixing, Zm equates with the mean depth of the loch (132 m), but even during summer stratification the morphometry of the loch and the strong prevailing winds produce a deep thermocline and an epilimnetic mixed layer of about 30 m or greater. Hence, throughout the year the quotient Zm/Zeu is exceptionally high and the underwater light climate particularly unfavourable for phytoplankton production and growth. 2. Phytoplankton biomass expressed as chlorophyll a is very low in Loch Ness, with a late summer maximum of less than 1.5 mg chlorophyll a m-3 in the upper 30 m of the water column. This low biomass and the resulting very low photosynthetic carbon fixation within the water column are evidence that a severe restraint is imposed on the rate at which phytoplankton can grow in the loch. 3. The chlorophyll a content per unit of phytoplankton biovolume and the maximum, light-saturated specific rate of photosynthesis are both parameters which might be influenced by the light climate under which the phytoplankton have grown. However, values obtained from Loch Ness for both chlorophyll a content (mean 0.0045 mg mm-3) and maximum photosynthetic rate (1–4 mg C mg Chla-1 h-1) are within the range reported from other lakes. 4. Laboratory bioassays with the natural phytoplankton community from Loch Ness on two occasions in late summer when the light climate in the loch is at its most favourable, suggest that even then limitation of phytoplankton growth is finely balanced between light and phosphorus limitation. Hence, for most of the year, when the light climate is less favourable, phytoplankton growth will be light limited. 5. Quotients relating mean annual algal biomass as chlorophyll a (c. 0.5 mg Chla m-3) and the probable annual specific areal loading of total phosphorus (0.4–1.7 g TP m-2 yr-1) suggest that the efficiency with which phytoplankton is produced in Loch Ness per unit of TP loading is extremely low when compared with values from other Scottish lochs for which such an index has been calculated. This apparent inefficiency can be attributed to suppression of photosynthetic productivity in the water column due to the unfavourable underwater light climate. 6. These several independent sources of evidence lead to the conclusion that phytoplankton development in Loch Ness is constrained by light rather than by nutrients. Loch Ness thus appears to provide an exception to the generally accepted paradigm that phytoplankton development in lakes of an oligotrophic character is constrained by nutrient availability.