Technical Abstract: The weevil, Ceratapion basicorne, which attacks rosettes of yellow starthistle, appears to be common in Central Turkey, attacking 25 to 100% of yellow starthistle plants. In a field experiment, weevils from the genus Ceratapion attacked 90% of yellow starthistle plants and 88% of milk thistle plants but not seven other plant species, including artichoke and safflower. We suspect that a different species of insect attacked milk thistle, but they emerged in the field before we could collect them for identification. Laboratory tests showed that C. basicorne does not oviposit in milk thistle. Ceratapion basicorne appears to be more host specific than was suggested by previous studies of a population in Italy. The insect is gregarious, and the number of larvae per plant was positively correlated to root diameter. We found up to 7 larvae attacking a single plant. The level of damage to individual plants was positively correlated to the proportion of plants attacked, indicating aggregation both among plants and within plants. Field data did not show any impact of the insect on plant size or number of flower heads, but germination rate of seeds produced by infested plants was 15% lower than for uninfested plants at two of three sites studied.