ABSTRACT: Emiliania huxleyi is a ubiquitous coccolithophore, capable of forming large blooms. This species presents a digenetic heteromorphic life cycle, the non-motile diploid phase typically bearing coccoliths and the flagellated haploid phasebeing non-calcified. Oxygen production rates at different irradiances of both phases were studied in mid-exponential and transitional growth phases in cultures grown under identical conditions. There were no significant differences in basic photosyntheticparameters (α^{chl a} , maximum light utilization coefficient; P^{chl a}_max, the light-saturated maximal rate of photosynthesis; E_k, the light-saturation parameter) betweenthe 2 life-cycle phases; however, whereas the diploid phase did not exhibit photoinhibition at irradiances up to 1000 µmol photons m^–2 s^–1, photoinhibition was recorded in the haploid phase above 400 to 500 µmolphotons m^–2 s^–1 and photosynthetic rate decreased to ca. 75% of P_max at 1400 µmol photons m^–2 s^–1. The 2 phases cultured under identical (non-saturating in terms oflight) conditions did not present any significant differences in pigment content. These results are discussed in an ecological context. The lack of photoinhibition could confer a competitive advantage on the diploid stage, notably in a turbulentenvironment. For the haploid stage, the occurrence of photoinhibition may indicate niche separation (spatial and/or temporal) relative to the diploid phase.