Seeding experiments in flow-through reactors were performed to investigate the incorporation of the opportunistic pathogen Pseudomonas aeruginosa into drinking-water biofilms on ethylene propylene diene monomer (EPDM) rubber as an example of an elastomeric material with a tendency to support biofilm formation. The continuous flow of tap water through the reactor resulted in the formation of biofilms which were in a quasi-stationary state after 14 days with average total cell counts of approximately 1.2 x 109 cells per cm2 and heterotrophic plate counts of about 1.0 x 108 colony-forming units (cfu) per cm2. After introduction of P. aeruginosa, these bacteria persisted in the drinking-water biofilms for at least 7 days and 5 weeks under stagnant and flow conditions, respectively. At the same time, P. aeruginosa was detected in the water phase, indicating detachment of cells from the biofilms. The extracellular polysaccharide alginate and cell-bound lectins LecA and LecB which are known to be involved in monospecies P. aeruginosa biofilm formation had no significant influence on the colonization of established drinking-water biofilms. From a health perspective, drinking-water biofilms can thus act as a reservoir for P. aeruginosa and have to be considered as a source of contamination in water distribution systems.