In 1999, we reported our discovery, in California and Oregon, of Chaetorellia succinea (Costa) (Diptera: Tephritidae) destroying the seeds of yellow starthistle, Centaurea solstitialis L., one of the worst weeds in the western United States. This fly, an unintentional introduction from Greece, dispersed rapidly throughout California and the northwest, and there is interest in using this adventive fly as a classical biological control agent for this weed. Because the host range of Ch. succinea has not been studied, this fly might pose a risk to other members of the thistle tribe Cardueae, especially the many thistle species native to California and other parts of the western United States. We determined the physiological host range of this fly in the laboratory by exposing it under no-choice conditions to 14 potential Cardueae hosts. Two introduced weed species and the native American basketflower (Centaurea americana Nuttall) were laboratory hosts. Under less restrictive choice test conditions, yellow starthistle was highly preferred, but there was a small amount of oviposition, and a few adult Ch. succinea emerged from all three of these plant species. Because Ch. succinea is now widespread throughout California, we collected flower heads from 24 potential host plant species at 111 sites to determine the realized host range in the field. These collections did not include American basketflower, which does not occur naturally in California. Ch. succinea emerged only from the other two known hosts: Ce. melitensis and Ce. sulfurea. Our results suggest that American basketflower growing in the southwestern United States may be at risk if Ch. succinea expands its range into that region.