Allozyme frequency data supported by colonization records and field observations were used to show that the Italian thistle (Carduus pycnocephalusL.) race ofRhinocyllus conicusFroelich recently has transferred to and adopted as new hosts two thistles native to California,Cirsium californicumGray andCirsium proteanumJ. T. Howell. Other races ofR. conicusestablished in California include weevils originating from Europe on the musk thistle (Carduus nutansL.) and milk thistle (Silybum marianum[L.] Gaertner).R. conicusfrom Italian thistles and musk thistles are genetically distinct from the milk thistle race displaying a Rogers' (modified) genetic distance of 0.3 units