Fungus-growing ants (Attini) are abundant and diverse, yet only one taxon of flies (Phoridae) and one of wasps (Diapriinae) are known parasitoids, and the biology of most species is not well known. Here we describe the first evidence for an ant parasitoid in the family Chloropidae (Diptera), in which larvae of Pseudogaurax paratolmos Wheeler, new species, parasitize larvae of the ant, Apterostigma dentigerum Wheeler, 1925. Larval flies are solitary ectoparasitoids, each of which attaches to a single ant larva and develops from larva to pupa in ~2 wk, consuming nearly the entire host, and then ecloses as an adult ~1 wk later. Overall parasitism prevalence was 6.8% of 203 nests, and flies were active during both the dry and rainy seasons. Intensity of parasitism ranged from 18.2 to 100% of larvae attacked per parasitized nest. No other species of Apterostigma that nested in the same localities were parasitized by the flies, including A. pilosum (Mayr, 1865) (n = 93 nests) and A. auriculatum (Wheeler, 1925) (n = 10 nests). All immature ants, parasitized or not, as well as immature stages of Pseudogaurax paratolmos, were attended by adult ants that exhibited normal brood care behavior, including covering immatures with mycelia, grooming, and maintaining brood in the fungus garden.