Chlorine dioxide has been proposed as an alternative to chlorine because it is a powerful disinfection agent that does not react with ammonia or chlorinated organics to form chloramines or trihalomethanes. The major reduction product formed when chlorine dioxide is added to water is the chlorite ion. Chlorite has been shown to be less toxic than free or combined chlorine. However, previous studies with eight freshwater families produced a US Environmental Protection Agency (USEPA) acute water quality criteria (WQC) lower than the WQC for chlorine. In the current study, an additional 12 families were added to the toxicological database to determine ecological risk criteria (ERC) using both the standard USEPA WQC methodology and the USEPA Office of Pesticide Programs (OPP) probabilistic approach. Chlorite LC50 values for the 20 family database ranged from 0.27 mg/L for the Daphnidae to 208.76 mg/L for the Salmonidae. The Daphnidae were by far the most sensitive family tested, with an LC50 44 times lower than the next most sensitive family tested, the Hyalellidae (LC50=1.19 mg/L) and 214 times lower than the most sensitive fish family tested, the Ictaluridae (LC50=5.79 mg/L). The Criteria Maximum Concentration calculated using the USEPA WQC methodology was 0.025 mg/L while the 95% ERC calculated using the USEPA OPP probabilistic methodology was 0.135 mg/L. The USEPA OPP probabilistic approach yields a criterion more consistent with the overall species sensitivity distribution and is not overly skewed by the low Daphnidae LC50 as is the USEPA WQC approach.