Photoinhibition of photosynthesis in natural populations of phytoplankton has been investigated in situ in Windermere, English Lake District, and Llyn Padarn, North Wales. The extent of inhibition was found to be directly related to irradiance and to increase with time of exposure. Inhibition was increased when the ultra-violet component of sunlight was not removed by enclosure of samples in glass bottles. Supersaturation with oxygen to the extent likely to occur in the surface waters of lakes did not appear to be an important factor increasing photoinhibition. Experiments on the effect of nutrient deficiency were inconclusive. It was demonstrated that populations could recover from severe photoinhibition within periods of less than 24 h and that when the water column is stable the population in the surface water adapts to become more resistant to photoinhibition.