SUMMARYPreadapted cultures were grown in a 12:12 LD cycle at a series of light intensities under cool‐white, fluorescent lamps.Pyrocystis fusiformisMurray maintained high division rates at low light intensities at the expense of cell size. In contrast,Dissodinium lunula(Schuett) Taylor had relatively lower division rates at low light intensities with little concomitant decrease in size. The response ofP. noctilucaMurray was intermediate between these two species. For all three, cell numbers did not increase above an intensity of 5–10 μEin·m−2·sec−1and division rate was saturated atca.30, 60, and 60μEin·m−2·sec−1forP. fusiformis, P. noctiluca, andD. lunula, respectively. The capacity for stimulable bioluminescence was saturated at light intensities of 0.15 μEin·m−2·day in short‐term (2‐day) experiments. In cultures ofP. fusiformisandP. noctiluca, maintained for at least one month at lower intensities than needed to saturate division rate, a decrease in the capacity for stimulable bioluminescence was accompanied by a reduction in cell size. Our results suggest that cell size and bioluminescent capacity may prove to be a potentially useful indication of the history of exposure of natural populations ofPyrocystiss