Field plot experiments were conducted to examine the interactions between tall fescue grass (Festuca arundinacea Schreb.), musk thistle (Carduus nutans L. = thoermeri Weinmann # CRUNU), and two thistle weevils Trichosirocalus horridus (Panzer) and Rhinocyllus conicus Froelich. Restriction of musk thistle growth was greatest when the weevils were allowed to feed on the musk thistles competing with tall fescue. Significant reductions were found in total musk thistle seeds per plant, root weight, flower buds per plant, stem dry weight, seeds per head, root crown diameter, stem height, rosette diameter, and head diameter. Seed weight and viability were not reduced. Of the three stress factors (tall fescue and the two Carduus thistle weevils), tall fescue had the greatest impact. When musk thistle seeds were planted in 1-yr-old tall fescue, germination was low. Seeds that germinated did not grow more than four Leaves and none of the thistles developed to the reproductive stage. A 1-year-old tall fescue stand effectively prevented musk thistle reproduction. Thus, tall fescue helped suppress musk thistle growth more quickly than the use of Carduus thistle weevils alone without competitive vegetation. Dry weight of tall fescue grass was lower in musk thistle-infested plots than in the thistle-free plots.