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Trupanea arizonensis Malloch is a monophagous, univoltine fruit fly (Diptera: Tephritidae) infesting flower heads of Trixis californica Kellogg var. californica (Asteraceae) in southern California. The egg, first- and third-instar larvae, and puparia are described and figured for the first time. As with each of the eight other Trupanea species previously studied, the lateral spiracular complex of the third instar is unique to T. arizonensis, comprising three verruciform sensilla on the meso- and metathorax and two verruciform sensilla on each abdominal segment. The third instar of T. arizonensis also differs from those of other Trupanea species previously studied in that both the meso-and metathorax are circumscribed by verruciform sensilla. The life cycle is of the aggregative type; whereby, the adults are long-lived and comprise the over-summering and over-wintering stage that returns with winter rainfall to aggregate on regrowing host plants for mating, and for oviposition during spring in the preblossom flower heads. The first instar tunneled into a single floral tube or ovule of a single immature floret, and each second and third instar continued its development by feeding principally on sap conducted to the excavated distal end of the same, then slightly stunted ovule. Pupariation occurs in the open flower heads, within which an average of 13% of the achenes were destroyed among heads containing puparia. Pteromalus sp. (Hymenoptera: Pteromalidae) was reared from individual puparia of T. arizonensis as a solitary, larval-pupal endoparasitoid.