In mid-1996, we detected an unintentionally introduced seed-head fly, Chaetorellia succinea (Costa), destroying seeds of yellow starthistle, Centaurea solstitialis L., one of the worst weeds in the western United States. In overseas studies, Chaetorellia succinea had been considered as a potential biological control agent for yellow starthistle, but had been rejected because of fears that it might become a pest of safflower, Carthamus tinctorius L., in the United States. From mid-1996 through early 2000, we conducted both laboratory and field evaluations to determine whether this fly could cause significant damage to safflower, a widely planted crop in California. In laboratory no-choice host range evaluations, adult females would oviposit, and the larvae completed development, on all five varieties of safflower that we tested. However, in choice tests, only one head each of two varieties of safflower was attacked. No safflower was attacked at three sites in California and Oregon, with large populations of Chaetorellia succinea, where we grew five varieties of safflower as ‘trap plants.’ Our monitoring of possible Chaetorellia succinea attack on safflower growing in 47 fields in California detected a small, but persistent population of this fly infesting an uncommon safflower variety at one field. We feel that our results indicate a minimal risk to commercial safflower growers, and this fly continues to show promise in assisting toward the eventual biological control of yellow starthistle.