Field studies were conducted at Rome, Italy, onCeutorhynchus trimaculatus(F.) andTrichosirocalus horridus(Panzer), two candidate insects for the biological control ofCarduusthistles in the United States. The biologies of these weevils were similar; teneral adults appeared in the spring and fed for 2 to 3 weeks on leaves of the primary host,Carduus macrocephalusDesfontaines, before estivation in the soil. Both weevils resumed feeding in the fall. Mating and oviposition occurred from November to March. Eggs ofC. trimaculatuswere laid on the surface and under the epidermis of the leaves, whereas eggs ofT. horriduswere laid in feeding grooves in the midrib. Larvae of both species fed together in the crowns and buds from December to April, but no cannibalism was observed. More larvae ofT. horridusfed in buds than did larvae ofC. trimaculatus. Pupation occurred in the soil. Both weevils were univoltine.