Dynamics in the production of extracellular polymeric substances (EPS) were investigated for the benthic diatoms Cylindrotheca closterium (Ehrenberg) and Nitzschia sp. The effect of growth phase and light:dark conditions were examined using axenic cultures. Two EPS fractions were distinguished. Soluble EPS was recovered from the culture supernatant and represented polysaccharides that were only loosely associated with the cells. Bound EPS was extracted from the cells using warm (30° C) water and was more closely associated with the diatom aggregates. Concentrations of EPS exceeded internal concentrations of sugar throughout growth, indicating that EPS production is important in these organisms. Soluble and bound EPS revealed distinct differences in daily dynamics during the course of growth. Soluble EPS was produced continuously once cultures entered the stationary phase. During the stationary phase, chl a-normalized EPS production rates equaled 6.4 and 3.4 d1 for C. closterium and Nitzschia sp., respectively. In contrast, production of bound EPS occurred only in the light and was highest during the exponential phase. Up to 90 of the attached EPS that was produced in the light was degraded during the subsequent dark period. The monosaccharide distribution of EPS was constant during the course of the experiment. The soluble EPS consisted of high amounts of galactose and glucuronic acid, relative to rhamnose, glucose, xylose/mannose, and galacturonic acid. In contrast, glucose was the dominant monosaccharide present in the bound EPS. These differences suggest that the production of the two distinct EPS fractions is under different metabolic controls and probably serves different cellular functions.