The existence of certain filamentous organisms may indicate one or more operation and control problems in activated sludge reactors. Microscopic evaluation of these filamentous organisms needs staining methods that can be achieved after some steps. This study is the first to show that textile dyes such as Astrazon Blue FGGL (BB41) and Astrazon Red FBL (BR46) can be used to identify filamentous organisms. Both dyes were used as a new, effective, and easy method for detecting the presence of filamentous bacteria. When contacted with BB41 and BR46, the filamentous bacteria and their characteristics were easily observed under the microscope. According to sources,textiledyesusedinthisstudyhavenotbeenused to diagnose filamentous bacteria before. According to the findingsof thisstudy, important morphologicalproperties of filamentous bacteria were easily observed using BB41 and BR46. “Attached growth, branching, septa, and granules of various nutrients” were clearly seen by the stains (BB41andBR46)withthehelpofthismethod.Thismethod may allow the examiners to examine the specimens of filamentous bacteria in the activated sludge reactors. Detailed information has been presented in this paper.