Carbon (14C) assimilation and the release of labeled dissolved organic carbon (DO14C) by Synura petersenii Korshikov (Chrysophyceae) were investigated at a range of irradiances (100–1260 μmol photons m−2 s−1) in axenic batch cultures. At up to 1260 μmol m−2 s−1, carbon assimilation was not saturated, although a 25% chlorophyll-a reduction in relation to the initial value at 1000 μmol m−2 s−1 and 28% at 1260 μmol m−2 s−1 was observed. The DO14C release was low at all irradiances investigated and dependent on carbon fixation; the highest release (4.8% of the total 14C assimilated) was observed at 1260 μmol m−2 s−1. In a second experiment, 14C assimilation by S. petersenii was measured at 130 μmol m−2 s−1. The subsequent loss of labeled particulate organic carbon (PO14C) and release of DO14C were determined during a 5-h period in which cells were incubated in nonlabeled medium at different irradiances. At 2000 μmol m−2 s−1 there was a 20.6% PO14C loss. The values of DO14C release were low for both 2000 μmol m−2 s−1 and 1000 μmol m−2 s−1 (4.9 and 4.6%, respectively). At lower irradiances, DO14C release was close to 2.0%. Preliminary analysis showed that the high molecular weight fraction released was an extracellular heteropolysaccharide.