The beetle Thamnurgus euphorbiae Küster, whose larvae feed in the inner stems of Euphorbia characias L., found in Italy, was selected as a candidate agent for the biological control of Euphorbia esula-virgata Waldstein and Kitaibel in the United States. Its adaptation to and survival on several E. esula-virgata ecotypes from North America justified host range studies conducted in Italy during 1993–2001. Of the 47 plant species or varieties in 13 families tested, the beetle fed and completed its development on the control plant E. characias L. and test plants E. esula L. and E. cyparissias L. from Italy and E. esula-virgata from Nebraska, Montana, Oregon, Colorado, and Idaho. The other plant species in the subgenera Agaloma, Euphorbium, Chamaesyce, and Poinsettia, and the economic test plants including Zea mays L. and Ficus spp., were unsuitable for the development of eggs, larvae, and adults. Plants in the Labiatae, Tamaricaceae, Ranunculaceae, and Tropaealaceae families (hosts of other Thamnurgus species) were tested, but no feeding or oviposition was observed. A laboratory cage test was carried out testing the most important ecotypes of leafy spurge to be controlled in the United States. T. euphorbiae attacked 60% of the exposed stems present on test plants and control plants. No significant difference was found between E. esula-virgata American ecotypes, the control plant E. characias, and the E. esula ecotype from Italy. T. euphorbiae adults attacked big stems measuring 3.1–4.1 mm in diameter. Smaller stems (1–2 mm in diameter) were not attacked. A petition for the introduction of the beetle into U.S. quarantine has been approved by the U.S. Department of Agriculture-Animal and Plant Health Inspection Service’s Technical Advisory Group for Biological Control Agents of Weeds in 1999. The release of the beetle in the field is dependent on additional host range studies with native and rare U.S. plant species requested by Technical Advisory Group for Biological Control Agents of Weeds. This paper reports results of host range testing conducted at the USDA-ARS-EBCL, Rome Substation, Italy, from 1993 to 2001.