A series of 3-halo-5-phenyl- and 3-phenyl-5-haloisoxazoles has demonstrated anthelmintic activity at doses ranging from 16 to 500 mg/kg orally against the rat roundworm, Nippostrongylus braziliensis. In the 5-phenyl series a halogen at the 3 position of the isoxazole ring was required for activity. However, in the 3-phenyl series activity was maintained after replacement of the 5-halogen with certain alkoxyl, thioalkoxyl, or amino groups. The 3-phenyl and 5-phenyl series apparently are not acting biologically at a common receptor site. Synthetic methods and structure-activity relationships are discussed.