The partial 16S gene sequences of six filamentous cyanobacterial strains, Oscillatoria lmosa KCTC AG10168, Oscillatoria princeps KCTC AG10153, Oscillatoria sp. KCTC AG 10184, Phormidium tenue KCTC AG10158, Phormidium parchydematicum KCTC AG10164, and Lyngbya hieronymusii KCTC AG10199, which were isolated in the late summer at Daecheong Reservoirs, were determined and assigned their phylogenetic and taxonomic position among taxa of order Ocillatoriales whose partial 16S rRNA gene sequences aligned in this suty, were very heterogeneously clustered with other taxa. The two strains, Oscillatoria limosa KCTC AG10168 and O. princeps KCTC AG10153, formed a cluster with O. sancta PCC7515, which supported 64% of the bootstrap trees with high similarity (19-96.15%). Strain Oscillatoria sp. KCTC AG10184, that was known to produce a nasty substance, was closely related to the toxic Oscillatoria group. The study on morphological variation in various environments and toxin production will confirm the taxonomic status of these species. Phormidium tenus KCTC AG10158 and Phormidium parchydematicum KCTC AG10164 made a cluster with other oscillatorian species of Phormidium, Oscillatoria, and Leptolynbya, which supproted 100% of the bootstrap trees with a very high sequence smilarity (96.8-99.8%) in thsi study. The sequence analysis in this study also supported that taxa of oscillatoriales are not monophyletic. Some of the fractures, such as the presence or absence of sheath and cell shape, which were used to define them, would be inadequate and should be reconfirmed. We suggest that sequences of partial 16S rRNA gene fragments aligned in this study should be more useful than morphological features in the identification and reconfirmation of the taxonomic status of these oscillactorian cyanobacteria.