The effect of photoinhibition on productivity was measured by the use of homocontinuous cultures of the marine diatom Phaeodactylum tricornutum which were kept at constant chlorophyll content by automatical dilution. The controls were illuminated continuously with 19 µE m^-2s^-1, whereas the photoinhibited culture was treated additionally with strong light of 2900 µE m^-2s^-1 for a period of 2 and 5 hours per day, respectively. The production was measured on the basis of the dilution volume per time and on the basis of chlorophyll, cell number and dry weight. Photoinhibition was characterised on the basis of changes in variable fluorescence and quantum yield and by light saturation curves measured at different stages during high light treatment and consecutive recovery. Although the strong light treatment reduced, the variable fluorescence to about 20% and the apparent quantum yield for the oxygen evolution was decreased to about 30%, the productivity of photoinhibited cultures was exactly the same as that of the controls. After several inhibition cycles the photoinhibited cultures showed significant increase in oxygen evolution and decreased susceptibility to photoinhibition. However, the results also indicate that photostress modulates the carbon acquisition efficiency under light saturation.