The susceptibility of toxigenic Aeromonas spp. to free chlorine in drinking water supplies, and the influence of environmental temperature on the bactericidal activity of the oxidant, were evaluated. The results showed inactivation curves characterized by an initial phase of rapid reduction of viable cells followed by a slow inactivation of bacteria. The effect of the chlorine compound was markedly influenced by water temperature. At a summer water temperature (20 degrees C), the efficacy of the chlorine concentrations tested was found to be two to three times lower compared to that found at a winter temperature (5 degrees C). Resistance was moderately, but significantly, greater in Aer. hydrophila vs Aer. caviae and Aer. sobria, but all Aeromonas spp. were more susceptible than Escherichia coli. Selective pressure with free chlorine did not produce Aeromonas cells with higher levels of chlorine resistance.