The partitioning of photosynthetically fixed carbon (C) into different fractions of intracellular and extracellular C pools by an axenic culture of Cylindrotheca closterium (Ehrenberg) and a field sample of natural benthic diatoms was studied using short-term incubation with 14C at different irradiances. Hence, excretion was directly dependent on the level of irradiance. During the incubations, a comparable amount of about 70 and 75% of the fixed C, respectively, was excreted by the culture and the field sample. This excreted C was distinguished in 2 operational fractions of attached (closely bound to the cells) and colloidal (soluble) material. In the field sample, the percentage of excreted C decreased at irradiances higher than 300 µmol photons m-2 s-1, but it was constant for the culture over the whole range of different irradiances applied. The percentage of extracellular polymeric substances (EPS) in the attached and colloidal material, as obtained by ethanol precipitation, was constant over the range of the provided irradiances for both samples. Subsamples of natural benthic diatoms were treated with an antibiotic cocktail in order to exclude bacterial activity which resulted in unexpected higher values of incorporated C in the fractions of total C, intracellular C, colloidal C and EPS.