In higher plants non-photochemical dissipation of excess light, trapped by the pigment pool of photosystem II, prevents photodamage to the photosynthetic apparatus. We report here that an algal virus infecting Chlorella strain Pbi induces non-photochemical quenching of photosystem II fluorescence, indicating enhanced loss of absorbed light energy from photosystem II. This phenomenon occurs soon after the establishment of the virus infection cycle and is observed at low irradiance (20 micromol quanta m-2 s-1). At low light, infection associated non-photochemical quenching is not linked to extensive conversion of violaxanthin to antheraxanthin and zeaxanthin. However, such conversion occurs rapidly (2-10 min) in infected cells under conditions of high irradiance (100-300 micromol quanta m-2 s-1). Under similar conditions uninfected Chlorella cells do not display significant changes in non-photochemical quenching.