The efficiency of ultraviolet (UV) disinfection and the regrowth potential of total coliforms were investigated in humic waters. Experiments were conducted according to an experimental design in which a UV radiation dose range of 68-681 mW s/cm(2) was applied to waters containing various concentrations (0-10 mg/L) of fulvic acid. Experimental results strongly suggested that the harmful effect of UV radiation on bacteria was diminished in humic waters due to absorption of UV light. Increasing concentrations of fulvic acid appeared to enhance its influence with elevated doses of UV radiation. Measured inactivation responses were on the order of 1-6 log(10) units for the UV dose range used. Comparison of k (inactivation coefficient) values for nonhumic water to highly humic water indicated that k decreased proportionally as the fulvic acid concentration of water was increased. The results of dark-incubation tests indicated significant regrowth of bacteria in fulvic acid-containing waters.