The contamination of triclosan, which is a widely used antimicrobial agent, is of emerging concern for its potential toxicity to aquatic organisms and human beings. Chlorination, UV254 irradiation and ozonation are the main disinfection methods used in water treatment processes. Although studies have shown that triclosan could react with free chlorine and ozone, and undergo photolysis under UV irradiation, few of them focused on the effects of disinfection processes on the biotoxicity of triclosan. In the present study, the potential risk of triclosan in water before and after laboratory-scale disinfection processes, including chlorination, UV254 irradiation and ozonation, was evaluated by bioassay. The results showed that both acute toxicity and genetic toxicity of triclosan was increased by chlorination and UV254 disinfection but decreased by ozone disinfection. In other words, ozonation would be a preferential disinfection method for triclosan-containing surface waters. This finding will help us to choose an appropriate disinfection method for water treatment. Furthermore, it is proved that bioassay could be a feasible way for risk evaluation when concentrations of antibiotics in surface waters or drinking waters are very low.