Abstract Two rivers situated in Central Poland, the Bzura and the Pilica, were selected for analysis of their water quality using benthic diatom species as indicators. The Bzura River has previously been strongly contaminated with organic pollutants, while the Pilica River has been classified as having good water quality. Samples were collected from nine sites along the Bzura in April 2003 and seven sites along the Pilica in April 2006. The main aims of the study were to determine the dominant diatom species present in the rivers and to assess their usefulness as biological indicators. Water quality of the rivers was estimated on the basis of the SPI - Specific Pollution sensitivity Index (CEMAGREF 1982) and the GDI - Generic Diatom Index (Coste, Ayphassorho 1991) while trophic status was estimated using TDI - Trophic Diatom Index (Kelly, Whitton 1995). Indices determining saprobic water pollution (SPI, GDI) indicated water quality of class III and IV for the Bzura River and class II and III for the Pilica River. The trophic status determined on the basis of the TDI index revealed the Bzura water to be classified in the eutrophic-to-hypertrophic zone and that of the Pilica River to the oligo-mesotrophic-to-eutrophic zone. The diatom taxa that dominated in the Bzura River were species that are generally considered to be tolerant and resistant with respect to organic water pollution, including: Cyclotella meneghiniana Kützing, Gomphonema parvulum (Kützing) Kützing, Nitzschia palea (Kützing) W. Smith, Nitzschia paleacea Grunow, Sellaphora pupula (Kützing) Mereschkovsky, Stephanodiscus hantzschii Grunow, Ulnaria ulna (Nitzsch) Compere. In the Pilica River diatoms from groups that are sensitive and tolerant to organic pollution were seen to dominate, such as: Achnanthidium minutissimum (Kützing) Czarnecki, Aulacoseira granulata (Ehrenberg) Simonsen, Cocconeis neodiminuta Krammer, Cocconeis placentula Ehrenberg, Cocconeis placuntula var. lineata (Ehrenberg) Grunow, Cyclotella radiosa (Grunow) Lemmermann, Fragilaria crotonensis Kitton, Geissleria decussis (Øestrup) Lange-Bertalot et Metzeltin, Melosira varians Agardh, Navicula reichardtiana Lange-Bertalot, Planothidium frequentissimum (Lange-Bertalot) Lange-Bertalot, Pseudostaurosira brevistriata (Grunow) Williams & Round, Rhoicosphenia abbreviata (Agardh) Lange-Bertalot and Staurosira pinnata Ehrenberg.