Cultures of Ceratium fusus were grown at seven light intensities <5 to 400 μmol photons m-1 s-1 as photosynthetically active radiation (PAR)> in the laboratory. Measurements of bioluminescence capacity (BCAP), division rate, and cell size were made at each intensity and compared to similar data from natural populations. In cultures, BCAP increased by a factor of 5--10 with increasing light. Division rates ranged from 0 d-1 (5 μmol photons m-2 s-1) to about 0.16 d-1 (100 μmol photons m-2 s-1). In these cultures, BCAP was highly correlated with both log of light (r2=0.89) and division rate (r2=0.69), suggesting that BCAP may be of use as an in-situ indicator of division rate. In both cultures and natural populations, BCAP was poorly correlated with cell size (r2≈0.3 and r2≈0.1 respectively). For populations from the Gulf of Maine (~42 ¿N, 69 ¿W) and the Marine light-Mixed Layers (MLML) site (~59 ¿N, 21 ¿W), division rates in the mixed-layer could only be determined with statistical certainty at the Gulf of Maine station (0.04 to 0.17 d-1). At the MLML site, BCAP in C. fusis was greater in the mixed-layer in May and below the mixed-layer in August. The August result is opposite to what cuture photoenhancement studies would predict if light was the factor controlling BCAP. ¿ American Geophysical Union 1995