Four laboratory-scale sequencing batch reactors (SBRs) were operated to evaluate whether bioaugmentation with Acinetobacter spp. can be used to improve start-up and performance of enhanced biological phosphorus removal (EBPR) systems. Two of the SBRs were bioaugmented during start-up by adding pure cultures of Acinetobacter spp., the third reactor received an amendment of activated sludge from a laboratory-scale EBPR system, and the fourth reactor, receiving no amendment, served as a control. Various chemical parameters were measured to monitor the performance of the four SBRS. Oligonucleotide probes of nested phylogenetic specificity were designed to quantify the contribution of Acinetobacter to EBPR. The probes were characterized for use in quantitative membrane hybridizations and fluorescent in situ hybridizations. Data from hybridizations with samples collected from the SBRs show declining levels of Acinetobacter spp. over the experiment. All four reactors achieved significant phosphorus removal and 90% nitrification after three days of operation. The results do not show a positive correlation between levels of Acinetobacter and successful EBPR. (C) 1998 IAWQ. Published by Elsevier Science Ltd.