Flower heads and peduncles of musk thistle,Carduus thoermeriWeinmann, and flower heads of plumeless thistle,Carduus acanthoidesL., infested withRhinocyllus conicusFroelich were collected to study parasitism of the different developmental stages of the weevil in 1978 and 1979. No parasites of eggs or adults were found. Major larval-pupal parasites wereBracon mellitor(Say) (Braconidae),Nealiolus curculionis(previously known asAliolus curculionis) (Fitch) (Braconidae),Zatropissp. (Pteromalidae), andNeocatolaccussp. (Pteromalidae).B. mellitor,N. curculionis, andZatropissp. attackedR. conicusover a 4-week period; parasitism was greatest during the first 2 weeks of July. Rate of parasitization differed between locations on thistle plants, thistle species, sites with different periods of weevil establishment, years of collections, and sample date. Although high levels of parastization sometimes occurred, the overall rate was low.