Using oligonucleotide probes directed at the rRNA of filamentous bacteria, this study looks at the influence of the components of transient substrate overloads on the growth of the dominant filamentous bacteria of activated sludge fed by a synthetic substrate. By dissociating the massive input of organic matter from the oxygen shortage that the latter generally induces, it is revealed that each of these factors applied alone, induces only transitory, small-scale growth of the filaments Nostocoida limicola, Haliscomenobacter hydrossis. Thiothrix and of type 021N. In contrast, combining them during a reconstituted transient substrate overload with an artificially created oxygen deficit, induces very fast growth of H. hydrossis which is responsible for establishing major proliferation. This massive proliferation was easily reduced by chlorination.