Ames mutagenicity assay has been used to evaluate a small scale water treatment process which consists of bed filtration, sand filtration and chlorination process. Raw river water, bed filtered and sand filtered water demonstrated a negative mutagenic response with the Ames assay. Chlorinated water, however, induced mutagenicity suggesting chlorination produces increased levels of mutagenic activity in the treated water. Tester strains TA98 and TA100 which contain R-factor plasmid was found to be sensitive for the raw river water and chlorinated drinking water while TA1537 and TA1538 strains were not sensitivity for the studied waters.