The trihalomethane (THM) levels were examined in discharged water from a human waste treatment plant, in tap water and in the water of swimming pools. The amounts of residual free chlorine, consumed KMnO(4) and coliform in the water of swimming pools were also determined. A significant increase in the THM content was found in the water of swimming pools, where the proportion of chloroform against total THM was remarkably high. The high proportion of THM may have resulted from THM formation due to repeated disinfection by chlorine. The trihalomethane content increases in accordance with residual free chlorine in the water of swimming pools. When chlorine was under the standard level (0.4mg/l), some pools had high levels of coliform, but few had high concentrations of THM. However, when chlorine was over the standard level no pools had unsatisfactory levels of coliform, although more pools had high concentrations of THM.