Evolution of a filamentous bacterial population was monitored on four wastewater treatment pilot plants subject to stresses which have consisted of oxygen deficiencies and/or loading shocks. Fluorescent in situ hybridization (FISH) was used to perform filaments identification and quantification. Results obtained on the different pilot plants have led us to conclude that independently of the nature of the stresses, a single filamentous bacteria species was involved in the increase of the sludge volume index associated to the filamentous growth. In addition, when serial stresses were used, substitutions in dominant filamentous populations occurred: if another filament began to proliferate it caused the regression of the one which was formerly dominant.