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Trupanea actinobola (Loew) is an oligophagous, bivoltine fruit fly (Diptera: Tephritidae) infesting flower heads of Acamptopappus, Amphipappus, Bailyea, Chlorocantha, Gutierrezia, Psilostrophe, and Trichoptilium spp. (Asteraceae) in southern California, and other plant genera throughout much of the United States. Two different-sized morphs are reported from California; the smaller one and subject of the present paper, is associated with Acamptopappus, Amphipappus, and Gutierrezia spp. in the subtribe Solidagininae of the tribe Astereae; the larger is associated with Bailyea, Psilostrophe, and Trichoptilium spp. in the subtribe Gaillardiinae of the tribe Helenieae. An uncommon, apparently intermediate-sized, presumably different biotype reared from flower heads of C. spinosus, a plant introduced into southern California from Mexico and Central America, may belong to a different, eastern United States’ form of T. actinobola with different host affinities in the tribe Astereae. Trupanea actinobola sensu late has now been reported from five tribes, eight subtribes, 14 genera, and 27 species of Asteraceae. The egg, first-third instars, and puparium obtained from Acamptopappus sphaerocephalus (Harvey and Gray) Gray are described and figured. The third instar larva bears a pair of elongated integumental petals dorsomediad of the mouth hooks, which have not been seen in the eight other California congeners described to date. The metathoracic lateral spiracular complex consists of a verruciform sensillum, and the abdominal lateral spiracular complex consists of a placoid-type sensillum; thus the lateral spiracular complex is the only morphological. feature observed to differ among third instars of each of nine species of Trupanea studied to date. The life cycle of T. actinobola in southern California is of the aggregative type. Eggs are laid in closed, preblossom flower heads and inserted through the overlapping phyllaries. First instars feed on the ovules and florets, second instars on ovules, and third instars principally on the soft achenes. The receptacles of infested flower heads are not pitted by larval feeding. Four species of chalcidoid Hymenoptera were reared from individual puparia and mature flower heads bearing puparia of T. actinobola as solitary, primary, larval-pupal endoparasitoids: Eurytoma veronia Bugbee (Eurytomidae), Halticoptera sp. (Pteromalidae), Mesopolobus sp. (Pteromalidae), Pteromalus sp. (Pteromalidae).