We examined the mechanisms related to the diel variations in the parameters of the relationship between the rate of carbon fixation of phytoplankton and irradiance (P vs. E curve). Our goal was to understand what determines the phase of these variations relative to that of the light cycle. We grew the marine prokaryote Prochlorococcus in an axenic cyclostat culture system under a light-dark cycle that mimicked natural conditions at sea surface and followed changes in cell physiology with a 2-h resolution. Individual cells divide mostly in phase with each other, once a day at the beginning of the dark period. The quantum yields of chlorophyll fluorescence, the maximum quantum yield of carbon fixation and the maximum rate of carbon fixation (P-max(B)) exhibited diel variations over about factors of 2, 4, and 4, respectively, with maxima at the beginning of the light period. The morning drop in phi(Cmax) and the quantum yield of fluorescence, which was accompanied by only a small decrease (< 15%) of photochemial efficiency of PSII (F-v/F-m), suggests regulation by light and preceded the drop in P-max(B) by 4 h. The decrease in P-max(B) during the day matched a decrease in the transcription level of Rubisco. The quantum yield of fluorescence, phi(Cmax), and P-max(B) increased again during the dark period, but this recovery was slowed at the time of cell division. Our results suggest that the pattern of diel variations in the photosynthetic parameters is determined both by photoacclimation and the cell-division cycle.