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The population dynamics and parasitoid assemblages of two leafminers, Liriomyza helianthiSpencer and Calycomyza platyptera(Thomson) (Diptera: Agromyzidae), were examined over a 3-year period. Leafminers were sympatric and occurred together on the same two host plants, Helianthus annusL. and Xanthium strumariumL., at our study sites in northern California. Sixteen species of parasitoids were reared from the two leafminers and mortality from parasitoids was substantial, averaging about 74%. The most common species, such as Diglyphusspp. and Neochrysocharis arizonensisCrawford, show no bias in association with a particular leafminer or plant species. Of the remaining parasitoids, most were usually associated with a particular leafminer species (e.g., Opius dimidiatus[Ashmead] on L. helianthi). Only one parasitoid of the seven most common taxa (Chrysocharis ainslieiCrawford) was strongly associated with a particular host plant species (H. annuus) in either leafminer host. In addition, a review of the literature found that parasitoid assemblages in natural, unmanaged habitats tended to be more species rich than assemblages of parasitoids on leafminers in agricultural habitats, although significant heterogeneity occurs among studies.Nevertheless, the most common genera of parasitoids attacking leafminers (Diglyphus, Chrysocharis, Neochrysocharis, Opius) are prevalent in both habitats. Given the similarity in assemblages of many of the parasitoids attacking leafminers in managed and natural habitats and frequent proximity of the two habitats, there exists the potential for exchange of natural enemies among habitats, potentially enhancing pest control in agricultural areas.