Overwintering mortality of cagedRhinocyllus conicusFroelich, a weevil successfully introduced from France for the biological control ofCarduusthistles in Virginia, was studied over 4 consecutive winters. Mean mortality was 52.0±6.3%. Males suffered slightly lower mortality than females, but the difference was not significant. Prolonged caging as well as excessive moisture in cages caused significantly (P>0.01) increased mortality. Preferred overwintering sites were the thistle plant and its vicinity, cage corners, and excised thistle heads.