To explore the relationship between mixing dynamics in the surface ocean and picophytoplankton optical properties, we measured the distributions of single cell fluorescence in Prochlorococcus populations throughout a diel cycle. Two time series were conducted in the N. Atlantic, and both showed a shoaling of the mixed layer, due to surface warming or a rain-formed surface layer. These dynamics, coupled with the diel cycle of solar irradiance, drove the development of a depth gradient in mean red fluorescence of Prochlorococcus, due to photoacclimation, in the newly-stratified layer. Furthermore, the frequency distribution of single-cell fluorescence within field populations appears to have responded to changing mixing and photoacclimation dynamics, with photoacclimation in the absence of strong mixing generally resulting in a reduced variance in fluorescence within sample populations. Nighttime mixing in the absence of photoacclimation reversed this process and resulted in increased variation of single-cell fluorescence. Departures from normality in observed distributions suggest vertical mixing time-scales that are slightly longer than time-scales of photoacclimation. The behavior of the mean and variance, and possibly the third and fourth moments, of single-cell optical properties is consistent with photoacclimation in response to the physical dynamics. (C) 2000 Elsevier Science B.V. All rights reserved.