Hydrellia pakistanaeDeonier, a leaf-mining ephydrid fly, has been reported only from hydrilla,Hydrilla verticillata(L.f.) Royle, in the Punjab region of Pakistan and Karnataka State in southern India. Host range studies were conducted in a quarantine laboratory in Florida with a fly colony from India. Females tested on 29 plant species in choice tests oviposited on all, but preferred hydrilla. Fifty-one plant species were included in larval no-choice tests, resulting in small numbers of adults being produced on six species. Curly leaf pondweed,Potamogeton crispusL., an associate of hydrilla in Asia and an introduced weed in the United States, produced the most adults. No more than 4% of the larvae developed to adults on two native pondweeds, and none developed on four others. Only 4% of the larvae developed to adults onP. crispuswhen hydrilla was present in choice tests. A dramatically declining fly population was maintained onP. crispusuntil the seventh generation, when it died out. These studies indicated thatH. pakistanaewas safe to release in the United States for biological control of hydrilla.