The flowerhead weevilRhinocyllus conicus(Froelich) was introduced from Europe into California in the early 1970's for biological control of weedy thistlesCarduus nutansL.,Carduus pycnocephalusL., andSilybum marianum(L.) Gaertn. (Cardueae). In this study, adultR. conicuswas reared from flowerheads collected from natural populations of 12 species of native, nontargetCirsiumthistles in California:Cirsium andersonii(Gray) Petrak,C. brevistylumCronq.,C. californicumGray,C. campylonH. Sharsm.,C. callilepis(Greene) Jeps.,C. ciliolatum(L. Henders.) J. T. Howell,C. cymosum(Greene) J. T. Howell,C. douglasiiDC.,C. hydrophilum(Greene) Jeps. var.vaseyi(Gray) J. T. Howell,C. pastorisJ. T. Howell,C. proteanumJ. T. Howell, andC. tioganumCongd. These are new host records for this weevil. This extension of its host range is not surprising because EuropeanCirsiumwas known to be within the host range ofR. conicus. The impact, if any, of these new host associations is not yet known. The weevil was also reared from the naturalized weedy thistlesC. nutans,C. pycnocephalus,C. tenuiflorusW. Curt.,Cirsium arvense(L.) Scop.,Cirsium vulgare(Savi) Tenore, andS. marianum. Quantitative rearing data are reported for each thistle host.