Gall midges (Diptera: Cecidomyiidae) comprise a diverse family of small nematoceran flies. While most species are phytophagous, the family also includes predatory species, and endoparasitoids. Endoparasitic species have been reared from aphids and psyllids. We discovered parasitoid-infected weaver ant, Oecophylla longinoda (Latreille) (Hymenoptera: Formicidae) in nests on mango trees in coastal Kenya. Examination of 5 colonies revealed the presence of parasitoids in all of them, with parasitism rates ranging from 2 to 21%. Morphological examination of larval parasitoids indicated that they were gall midges, and molecular data were consistent with the identification. Attempts to rear adults were unsuccessful. This study presents to the best of our knowledge the first report of Cecidomyiidae parasitising Hymenoptera, as well as the first report of a parasitoid of weaver ants. This species should be investigated further because its damage may limit the ability of weaver ants to control pests of cultivated fruits and other insects.