The occurrence of the biological control weevil Rhinocyllus conicus is documented for the first time on the non-target thistle species, Cirsium hillii, in Wisconsin. Rhinocyllus conicus was released locally twice in Wisconsin before studies showed non-target compatibility with the rare C. hillii and the federally threatened C. pitcheri. These previously unpublished studies from the Wisconsin Department of Agriculture, Consumer, and Trade Protection prevented further releases of R. concnicus in Wisconsin during the early 1980s. Despite only localized releases, data from state collection records indicate that R. conicus has spread at least 80 miles from the initial release location. While R. conicus's effects on other non-target Cirsium species are well documented, the lack of information on the biology of C. hillii precludes understanding the effects of R. conicus on this species' conservation.