Several species of Ulidiidae (Diptera) are primary pests of maize (Zea mays L.). The ability to distinguish their immature stages would be useful for biological studies where their distributions overlap. Morphology of the immature stages was examined for three Ulidiidae that attack maíze in the southeastern United States: Chaetopsis massyla (Walker), Euxesta eluta Loew, and Euxesta stigmatias Loew. Egg, larval, and pupal characters were measured and described with the aid of light and scanning electron microscopy. Because of considerable overlap in character states, only a few traits in each stage could be used to separate these three species. C. massyla eggs had pores restricted to the posterior end, but the pores were evenly distributed in the two Euxesta species. Eggs of the two Euxesta species could not be differentiated. Larval mouth hooks of C. massyla had a distinct tooth on their ventral surface, whereas this tooth was lacking in E. eluta and E. stigmatias. Fewer oral ridges were observed on C. massyla larvae than on E. eluta or E. stigmatias. Posterior spiracular slits were apparent in E. stigmatias versus obscure in E. eluta larvae. The length and spinule arrangements on creeping welts could be used to separate E. eluta and E. stigmatias larvae. Posterior spiracular plates of C. massyla puparia were trapezoidal versus ovoid in the two Euxesta species. Puparium color varied significantly among the three species.