Chaenusa glabra Kula, new species from India and Pakistan is described, and sexual dimorphism is reported and discussed. A diagnosis is provided to differentiate it from all other species of Chaenusa Haliday sensu lato. It was reared from Hydrellia pakistanae Deonier and Hydrellia sarahae laticapsula Deonier infesting Hydrilla verticillata (L. f.) Royale during foreign exploration for natural enemies of H. verticillata, and is in quarantine at the Florida Biological Control Laboratory in Gainesville. It is the first species of Chaenusa described from the Oriental Region and the only species of Chaenusa known to attack species of Hydrellia Robineau-Desvoidy infesting plants from a genus other than Potamogeton L.