Many fishes native to the Gila River Basin, Arizona, are on the decline with about 70 percent of the 17 fish species Federally listed as endangered or threatened. The decline has been partly attributed to the introduction of nonnative fishes that are of recreational interest such as catfish and smallmouth bass. Effective management practices are needed to control the nuisance nonnative fishes in Southwestern United States watersheds to prevent further decline of the native species and facilitate their restoration. An effective approach is the use of chemical toxicants to control the nuisance species. One chemical mixture of interest, Supaverm®, a combination of mebendazole and closantel, has been reported to show selectivity toward nonnative fish species of concern. We conducted acute toxicity tests on native and nonnative fish species of the Gila River (Arizona). Our findings showed that Supaverm® was not selectively toxic to the nonnative fish species suggesting that the use of the chemical mixture to eradicate those fish would not be effective.