Picophytoplankton dominate the phytoplankton community in wide ocean areas and are considered efficient in the acquisition of light compared to other phytoplankton groups. To quantify their photophysiological parameters we use three strains of picoprokaryotes and four strains of picoeukaryotes. We measure the acclimated response of the exponential growth rates and chlorophyll a (Chl a) to carbon ratios, as well as the instantaneous response of photosynthesis rates at 5-7 light intensities. We then use a dynamic photosynthesis model (Geider et al. 1997) and extend it with a photoinhibition term. We derive five photophysiological parameters: the maximum rate of photosynthesis (P C m ), the affinity to light (a chl ), the photoinhibition term (b chl ), the respiration rate (resp), and the maximum Chl a to carbon ratio (h max ). We show that P C m is significantly lower for picoprokaryotes than for picoeukaryotes and increases significantly with increasing cell size. In turn, a chl decreases significantly with increasing maximum growth rate (l max ). The latter finding is contrary to a previously reported relationship for phytoplankton, but agrees with theoretical assumptions based on size. The higher efficiency in light acquisition gives picoprokaryotes an advantage in light limited environments at the expense of their maximum growth rate. In addition, our results indicate that the accumulation of long-term damage through photoinhibition during acclimation is not well represented by the dynamic photosynthesis model. Hence, we would recommend to distinguish between the effects of irreversible damage (on a time scale of days) on growth rates and of reversible damage (on a time scale of minutes) on photosynthesis rates.