The results of a survey of parasitoid wasps associated with herbivorous insects on Sagittaria latifolia Willd. and Sagittaria platyphylla (Engelm.) J. G. Sm. (Alismatales: Alismataceae) in the Nearctic Region are reported. The following 10 wasp species were reared from insects on S. platyphylla: Gonatocerus sp., Mestocharis tropicalis Yoshimoto, Syntomopus americanus Ashmead (all Chalcidoidea), Hexacola websteri (Crawford), Rhoptromeris sp. (both Cynipoidea), Alabagrus texanus (Cresson), Bracon sp. nr. mellitor Say, Cotesia diacrisiae (Gahan), Enicospilus glabratus (Say), and Opius (Tolbia) hoffmanni (Fischer) (all Ichneumonoidea). Three wasp species, S. americanus, Chaenusa psillosae Kula (Ichneumonoidea), and B. sp. nr. mellitor, were reared from insects on S. latifolia. All species except C. psillosae are reported as associated with a species of Sagittaria for the first time. Mesochorus discitergus Say is reported as a hyperparasitoid of C. diacrisiae for the first time and is also reported as associated with a species of Sagittaria for the first time. Opius (Tolbia) hoffmanni is reported as a parasitoid of Hydrellia sp. prob. nobilis (Loew); this is the first record of O. (T.) hoffmanni from a species of Hydrellia Robineau-Desvoidy (Diptera: Ephydridae). Mestocharis tropicalis, S. americanus, and O. (T.) hoffmanni are reported from Louisiana for the first time. Syntomopus americanus and O. (T.) hoffmanni are reported from Texas for the first time.