In this study the in situ behavior of biofilm development and detachment was continuously observed by a video camera. The model biofilms consisting of denitrifiers were formed on the flat surfaces of polyvinyl-chloride plates that were placed in a rectangular open-channel reactor. The effects of intrinsic biofilm properties on biofilm detachment, such as intrafilm gas-vacuoles formation, extracellular biopolymer (ECP) content, biofilm density and cavity formation at the biofilm/substratum interface were quantified using attached-biofilms onto flat substrata. The result indicated that there were three phenotypes of massive biofilm detachment (see Fig. 4) under the conditions of low shear stress of flow. The first type showed that a large fraction (approximately 1/4-1/3) of the biofilm was sloughed off as a whole layer from the substratum surface at a relatively early stage of biofilm formation. This type was primarily caused by the cavity formation and by the decrease of ECP content. Succeeding to the first type, the second type occurred as fragmentary detachments. The second phenomenon was attributable to the increase in biofilm buoyancy due to the gas-vacuoles formation within the biofilm. The third type took place as the combination of the first and the second types following the occurrence of the second type. In contrast, at a higher flow shear stress only the second type of detachment became prevalent throughout the progression of biofilm formation.