In this article, I analyse how photoadaption affects the photosynthesis-irradiance curve in phytoplankton. Four parameters are presumably affected by photoadaption: (1) the size of a photosynthetic unit; (2) the number of photosynthetic units per cell; (3) the average turnover time of a photosynthetic unit; and (4) the chlorophyll-specific absorption cross-section. Prezelin's well-known conceptual model of photoadaption deals with variation in size and number of photosynthetic units, but not with the other two parameters. Prezelin's model predicts that the photosynthesis-irradiance curve is differentially affected by variations in size or number of photosynthetic units. By contrast, my analysis shows that if photoadaptional variation in the turnover time is also accounted for, only the cellular chlorophyll concentration is relevant; variation in the size vs. number of photosynthetic units is immaterial. The photoadaptional response of the chlorophyll-specific absorption cross-section allows a simple description consistent with experimental data. I use this description to derive a single expression for the rate of photosynthesis as dependent on irradiance and the cellular chlorophyll concentration. This model for the photosynthesis-irradiance-curve accounts for the photoadaptional alterations of all four parameters. Copyright 1997 Academic Press Limited