Thirty six predominant colonies were isolated from the SBR and control reactor samples and reduced to 14 restriction fragment length polymorphism (RFLP) profiles by RFLP analysis. Although the colonies isolated in each bioreactor sample showed different RFLPs, some appeared in more than one (Ta-ble 2). The most frequent bacteria isolated from all bioreactors were members belonging to Paracoccus and Pseudomonas. The most widely represented phylum (11 members) was that of the Proteobacteria, with bacteria from the Alfa proteobacteria (Paracoccus),
Gamma-Proteobacteria (Pseudomonas and Aeromonas) and Beta-Proteobacteria (Comamonas) classes. The remaining bacteria are distributed in equal proportions among the Bacteroides (Flavobacterium) and Actinobac-teria phyla. PCR amplification experiments showed that all isolated denitrifying bacteria harbour the nosZ gene, responsible for the last step in the denitrification pathway (reduction of nitrous oxide to molecular nitrogen (data not shown).