Haloacetic acids (HAAs) are disinfection byproducts (DBPs) formed by the addition of chlorine to drinking water supplies. The Stage II DBP Rule requires monitoring at peak HAA sites in the distribution system, yet the fate of HAAs and other DBPs in distribution systems remains poorly understood. The main goal of this research is to improve understanding of HAA biodegradation and HAA-degrading bacteria in order to facilitate the development of models for predicting HAA fate in distribution systems, to better assess temporal and spatial variability in exposure to HAAs and to facilitate the development of HAA control strategies. The specific objectives of this American Water Works Association Research Foundation-sponsored research project are to: (1) enrich for and identify HAA degraders from various water distribution systems and other environments; (2) develop a rapid and highly sensitive technique for detecting and enumerating HAA degraders; (3) evaluate the HAA degradation kinetics of cultures enriched on MCAA, DCAA, or TCAA; (4) evaluate effects of environmental conditions on HAA degradation rate in both suspended-growth batch reactors and biofilm reactors; and (5) evaluate the prevalence of HAA degraders in distribution systems including samples of water and pipe wall biofilms. To date, we have attempted to enrich for HAA-degrading bacteria from: activated sludge, tap water collected from three distribution systems, and biofilms scraped from the interior walls of an iron water main. Batch experiments are being used to investigate biodegradation of HAAs by the successful enrichment cultures and selected isolates. Furthermore, molecular techniques are being used to identify HAA-degrading bacteria.