Purple locoweed,Astragalus mollissimusTorrey, a native rangeland weed, is considered one of the most destructive weeds in the western United States. The chemical swainsonine, a secondary plant compound found inA. mollissimus, causes physiological and morphological abnormalities in domestic cattle, sheep, and horses. In 1991,Cleonidius trivittatus(Say), a weevil native to North America, was found attacking a large population of locoweed in northern New Mexico. Four study plots were established at Amistad, NM, in 1992 and 3 plots were established at Gladstone, NM, in 1993. Twenty–five1.0–m2quadrats were sampled every 2 wk from 28 March to 26 September 1992 and every 3 wk from 15 April to 4 September 1993 to determine the bionomics ofC. trivittatusand its interaction with locoweed.C. trivittatuswas determined to have 4 instars. Stadium length was 10, 13, 27, 47, and 16 d for 1st, 2nd, 3rd, and 4th instar, and pupal stages, respectively, in 1992 and 19, 14, 22, 50, and 18 d for 1st, 2nd, 3rd, and 4th instar, and pupal stages, respectively, in 1993.C. trivittatuslays more eggs as plant size increases. Substantial locoweed mortality results from feeding by more than 2 weevil larvae per plant.