Phytophagous insects representing 7 orders, 31 families, and 87 species are reported as comprising the insect fauna ofAmbrosia acanthicarpaHooker (Compositae) in southern California. Most insects found attacking this native ragweed were euryphagous, ectophagous, sap– and foliage-feeding species. About a third reproduced on this plant. Fourteen (19%) of the 73 identified associates were stenophagous species, their host plants apparently confined to the tribe Heliantheae. About a third of these identified associates were minor or major pests of cultivars.