Phytophagous insects representing 6 orders, 27 families, and 55 species are reported as comprising the insect fauna ofAmbrosia chamissonis(Lessing) Greene (Compositae) in southern California. Most insects found attacking this native ragweed were euryphagous, ectophagous, sap- and foliage-feeding species. Nearly half reproduced on this plant. Nine (20%) of the 44 associates identified to species were stenophagous, their host plants apparently confined to the tribe Heliantheae. Ten (23%) of these were minor or major pests of cultivars.