The Argentine ant, Linepithema humile (Mayr), is native to South America and invasive in many parts of the world, including North America. Various morphological forms of L. humile exist in South America, and it is possible that some forms may comprise distinct species. In Brazil, L. humile is reported to be attacked by parasitoids in the genus Pseudacteon (Diptera: Phoridae). If Pseudacteon parasitoids attack the invasive form of L. humile, they may be candidates for biological control of this ant. We compared body size, scape length, and pronotal pilosity among populations of L. humile that were attacked by parasitoids and those that lacked parasitoids. Ants that hosted parasitoids at foraging trails in Brazil tended to have more pronotal hairs and shorter scapes than ants that did not host parasitoids in Brazil, Argentina, and North America. We used paired experimental trials to determine whether phorid parasitoids exhibit narrow host specificity or whether they will attack the invasive form of L. humile if given the opportunity. Parasitoids located their natural host form but did not locate either North American L. humile or morphologically similar ants from Brazil. Together these results suggest that various forms previously treated as L. humile are in fact separate species, that L. humile is not a natural host of the parasitoids studied here, and that these parasitoids are not candidates for biological control of Argentine ants in North America.