This WRC funded project has studied the appropriateness of the ABR (anaerobic baffled reactor) for on-site primary sanitation in low-income communities. A 3,000 L pilot reactor was located at the Kingsburgh wastewater treatment plant south of Durban, South Africa. Feed to the reactor was raw domestic wastewater containing a significant proportion of particulate organic matter. The compartments of the ABR were routinely monitored for pH, COD, and gas production, among other physical-chemical determinants. The microbial population in each compartment was analysed by fluorescent in situ hybridisation, using general oligonucleotide probes for eubacteria and archeae and a suite of 10 genera or family specific probes. Scanning electron microscopy was conducted on the sludge fraction of each compartment. Mixed fractions from each compartment were also analysed for health-related indicator bacteria (total coliforms and E. coli). Results indicated that methanogenesis was not occurring to the expected extent in the latter compartments, and that this was probably due to a hydraulic load limitation. This contrasted with earlier studies on industrial effluent, for which the organic load was exclusively in soluble form. Inactivation of health-related indicator bacteria was less than 1 log, indicating the need for an additional post-treatment of the effluent to protect community health.