An ecological study of Rhopalomyia californica Felt (Diptera: Cecidomyiidae) was conducted at the Jasper Ridge Biological Preserve (Stanford University) during 1982-83. Analysis of life-table data suggested that its predators and parasites played a major role in maintaining population density at comparatively low levels. The parasite guild consisted of seven hymenopteran species: Torymus koebelei Huber & T. baccharidis Huber (Torymidae), Zatropis capitis Burks & Mesopolobus (Pteromalidae), Tetrastichus sp. (Eulophidae), Eupelmus inyoensis Girault (Eupelmidae), and Platygaster californica Ashmead (Platygastridae). It is suggested that R. californica, when introduced without its natural enemies, may be an important biological-control agent against weedy Baccharis spp. in Texas and Australia.