The weevil Rhinocyllus conicus Froelich was introduced in Georgia, USA as a biological control agent of musk thistle, Carduus nutans L. (Asteraceae) in 1991. Musk thistle populations and R. conicus infestations were monitored over 10 yr at four sites in central Georgia. Musk thistle populations declined substantially as R. conicus populations increased at all sites, but thistle infestations remained problematic at some sites. A laboratory cage study found that R. conicus would lay eggs on flower buds of yellow thistle, Cirsium horridulum Michaux, bull thistle Cirsium vulgare (Savi) Tenore, and blessed milkthistle, Silybum marianum (L.) Gaertner in no-choice and choice tests with and without Ca. nutans. Carduus nutans was preferred over the other thistle species, but the other thistle species were acceptable for oviposition by R. conicus. One or more mixed field infestations of musk thistle and yellow, bull, or blessed milkthistles were sampled for R. conicus infestation. Rhinocyllus conicus was not recovered from capitula of C. vulgare or S. marianum, but R. conicus was found infesting capitula of the native nontarget thistle C. horridulum at multiple field sites from 1997 to 2001. At four sites in 2000 and 2001, R. conicus infestations in C. horridulum averaged 7.47% and 31.19% of flowerheads sampled. This is the first report of R. conicus attacking the nontarget thistle C. horridulum in the field, but the impact that R. conicus may have on C. horridulum populations is not known.