ABSTRACT: Microphytobenthos, mainly benthic diatoms, produce mucilages containing extracellular polymeric substances (EPS) on the cell surface. The ratios of particulate organic carbon (¹⁴C-POC) and total extracellular organic carbon (TEOC: thetotal of extracellular dissolved organic carbon [¹⁴C-EDOC] and colloidal organic carbon [¹⁴C-Colloidal-OC] extracted with EDTA) to total primary production (TPP) were measured using ¹⁴C-tracer to estimate the ratio foreach fraction of organic carbon photosynthetically produced by microphytobenthos and phytoplankton. TEOC for microphytobenthos ranged from 42 to 73% of TPP and was made up mostly of ¹⁴C-Colloidal-OC. The TEOC/TPP for phytoplankton ranged from1.5 to 22%. The ratio of ¹⁴C-EPS to ¹⁴C-Colloidal-OC for microphytobenthos was 41 to 53%, and for phytoplankton was 4.4 to 24%. Algal species in the transitional phase were used for experiments because microphytobenthosNitzschia hybridaeformis most abundantly produced TEOC and EPS in the transitional phase during growth in experiments studying the effect of the growth phase. By using EDTA for the extraction of extracellular organic matter, it becomes clearthat excretion of colloidal organic matter by microphytobenthos has always been underestimated. These results suggest that the large amount of TEOC containing EPS produced by microphytobenthos plays a more important role than secretion of phytoplanktonas a source of organic carbon for heterotrophic organisms in the intertidal-flat ecosystem.