A collaborative pilot study of the microflora on biological activated charcoal (BAC) filters employed in the tertiary treatment of drinking water revealed the principle bacterial genera to be Pseudomonas, Alcaligenes, Achromobacter, Bacillus, Micrococcus, Corynebacterium, Chromobacterium, Microcyclus and Paracoccus. The microbial population of the filters paralleled seasonal carbon dioxide production. Of particular interest were the effects of the BAC miroorganisms upon precursors of trihalomethanes (THMs). Mixed populations of BAC microorganisms were cultivated for 50 days in a mineral salts-humic acid medium. It was concluded that (1) the BAC microflora enhances the absorptive capacity of the filters; (2) chemico-physical and biological processes operate in concert to lower the concentration of precursors of THMs; and (3) few bacterial pathogens establish themselves on the filters.