Phytophagous insects representing 7 orders, 36 families, and 113+ species are reported as comprising the insect fauna ofAmbrosia psilostachyaDecandolle (Compositae) in southern California. Most insects found attacking this native ragweed were euryphagous, ectophagous, sap- and foliage-feeding species. About ½ reproduced on this plant. Seventeen (18%) of the 95 associates identified to species were stenophagous, their hosts apparently confined to the tribe Heliantheae. Thirty (32%) of these were minor or major pests of cultivars.Prospects for biological control of ragweeds in North America and abroad are discussed.