I examined ant attendance and its importance to larval survivorship in a facultatively myrmecophilous butterfly, Icaricia acmon (Westwood and Hewitson) (Ly­caenidae), in a population that uses two host plant species, Eriogonum compositum Dougl. and E. strictum Benth. (Polygonaceae). Third and fourth instar larvae of I. acmon were tended by three ant species: Tapinoma sessile (Say), Formica neogagates Emery, and an unidentified Formica species. Third instar larvae were tended less frequently than fourth instar larvae on both plant species, and T. sessile was the attendant ant species for a higher proportion of third instar than fourth instar larvae developing on E. compositum . Over the duration of the study, all switches of attendant ant species on individual plants were from early T. sessile attendance to later F. neogagates attendance. An exclosure experiment revealed that ant attendance had no significant effect on larval mortality.