Two new species of cecidomyiids were identified on Prosopis caldenia Burkart (Fabaceae) in Argentina: Rhopalomyia caldeniae Cornejo Martínez sp. nov., as the inducer of bivalve leaf galls, and the inquiline Dasineura oportunista Cornejo Martínez sp. nov. These galls are composed of the partial coalescence of two contiguous pinnulae that delimit a single larval chamber, where both species develop. Seven other species were reared from these galls as parasitoids and inquilines.
Keywords: Diptera, Caldén, Gall midges, Neotropical Region, Taxonomy.