Initial colonization and succession, floral composition and organic matter accumulation(productivity) of the periphytic microalgae were studied in the freshwater zone of the Naktong and Sonaktong River Estuary from November 1991 to March 1993. The periphytons were harvested in the 20-3Ocm and 120-130cm depth under water surface using three artificial substrates such as wood, acryl plate and marble stone. In the initial succession, the yellow-brown unicellular algae and filamentous blue-green algae such as Phormidium and Lyngbya gradually increased and Oedogonium colonized the wood surface secondarily. The weekly organic matter accumulation has a peak during 4th week after substrate settlement and decreased after 4th week. Algal productivity in the upper periphyton community was 20-145㎍ chl/10cm^2 and in the lower community was approximately 12-93% of the upper community. However, organic matter estimated with ash-free dry weight(AFDW) of lower periphyton has 1.1-2.4 folds of the upper periphyton indicating the great mucilage accumulation in lower community. Upper community maintained the diverse microalgae but the lower was predominated with periphytic and planktonic diatoms. Substrate specificity of the diatom flora was not observed except one species, Navicula saprophila, which colonized only the upper part of wood. The floral compositions of periphytic diatom were those of phytoplankton flora. However, blue-green and green algal floras of the periphyton were different from the plankton community.