We measured specific growth rates of Stephanodiscus minutulus, Nitzschia acicularis (diatoms), and Limnothrix redekei (cyanobacterium) under fluctuating and constant light in semi-continuous culture at 10uC, 15uC, and 20uC and under photoperiods of 6 h d21 and 12 h d21. Fluctuating light regimes simulated regular vertical mixing in lakes with a ratio of euphotic to mixed depth (zeu:zmix) of 1 and 0.5 on a cloudless day. Light fluctuations at zeu:zmix 5 1 decreased the growth rates of S. minutulus, N. acicularis, and L. redekei by 18% ,3 3%, and 29%, respectively, compared to constant light at the same daily light supply. Temperature had no effect on this decrease. Halving zeu:zmix (simulating deep mixing) had the same effect on growth as halving the photoperiod, demonstrating that these factors are cumulative. We introduce a simple empirical factor to adjust growth rates measured under constant light to account for fluctuating light. This factor is independent of temperature and photoperiod, applies over a range of zeu:zmix, and accurately describes present and published growth rates of several species. We show how to account for temporal variability of the light supply at different temperatures and photoperiods when predicting growth rates of phytoplankton. Phytoplankton experience a steadily changing light supply, due to, for exampl e, the course of sunlight throughout the day, varying cloud cover, wave reflections, and vertical transport within the mixed layer. Thus, light is a complex resource for phytoplankton, with both temporal components, like daylength and intensity fluctuations, and a quantitative component, the amount of light energy. In terms of phytoplankton growth, ‘‘light limitation’’ generally refers to limitation by the amount of light energy; however, this may not be so simple. During spring, light and temperature generally limit algal growth in lakes before nutrient limitation sets in (Sommer et al. 1986). However, there is evidence that it is not the daily amount of light