Although considerable study ofChysolina quadrigemina(Suffrian) andC. hyperici(Forster) has occurred in California, Australia, and Canada, these species have not been thoroughly studied in Idaho or the Pacific Northwest. Northern Idaho populations of the two leaf beetles were sampled periodically from four (three grassland and one forest meadow) sites infested with St. Johnswort during 1985 and 1986. The egg was the dominant overwintering stage at most sites and most larval feeding took place in the spring and early summer. Beetle population levels and trends varied from site to site.C. quadrigeminawas the dominant species in the grassland sites;C. hypericidominated in the forested site. The bronze color morph ofC. quadrigeminadominated over the other color morphs at all sites where it was present. Populations ofAgrilus hyperici(Creutzer) (Coleoptera: Buprestidae) also were sampled periodically from the same four St. Johnswort infestations during 1985 and 1986. Adults emerged during late May through July. Eggs were laid during July. The proportion of stems infested with eggs at the four sites ranged from 0 to 92%. However, mean larval numbers were always less than one per plant. Live and dead plants contained live larvae. Nearly one-half of the dead plants at one site had once been infested. Based on the larval infestation levels observed (live larvae and adult exit holes), particularly in dead plants, it appears thatA. hypericihas contributed significantly to St. Johnswort suppression in Idaho. Annual grasses (i.e.,Bromusspp.) and weedy forbs (i.e.,Centaureaspp.) constitute the major replacement vegetation at most of the St. Johnswort infested study sites in northern Idaho.