Photoacclimation was examined in the marine diatom Skeletonema costatum, which was subjected to reciprocal shifts between irradiances of 50 (low‐light) and 1,200 (high‐light) µmol photons m−2 s−1. Cell chlorophyll a and fucoxanthin contents were higher but diadinoxanthin and diatoxanthin contents lower in cells grown at 50 µmol photons m−2 s−1 than in cells shifted to 1200 µmol photons m−2 s−1. Cell carbon contents measured at the start of the light period were similar in both high‐light an low‐light treatments. However, by 6 h into the light period, the carbon contents in the high‐light cells were about twofold higher than in the low‐light cells. Dark respiration rates, dark Chl a synthesis rates, and dark cell‐division rates were greater in the high‐light acclimated cells than in the low‐light cells. Thus, there was a greater uncoupling of carbon assimilation from cell division during the day in the high‐light cells, but pigment synthesis and cell division continued in darkness. Cell‐specific, light saturated photosynthesis rates, and chlorophyll a specific lightlimited photosynthesis rates were constant during reciprocal shifts between growth irradiances of 50 and 1200 µmol photons m−2s−1. Thus, differences of photosynthesis versus irradiance curves between cells acclimated to high‐light versus low‐light could be accounted for largely in terms of changes in cell chlorophyll a contents. Although the chlorophyll a‐specific initial slope, αchl, was constant, the chlorophyll a‐specific light absorbtion coeffecient, achl, increased and the maximum quantum efficiency of photosynthesis (φm) declined following the shift to high light. The increase of achl was most likely due to a decreased package effect. The decline of φm was most likely due to accumulation of xanthophyll cycle pigments. Carbon‐specific, light‐saturated photosynthesis rates were lower in high‐light than in low‐light cells; this observation may indicate that control of light‐saturated photosynthesis shifts from enzymes of the carbon dioxide reduction cycle (Calvin cycle) in low‐light cells to the photosynthetic electron transfer chain in high‐light cells.