The host range of a Panamanian moth,Parapoynxprob.rugosalisMöschler, a potential biological control agent against the recently introduced hydrilla,Hydrilla verticillata(L.f.) Royle, was investigated by confining individual larvae (n= 535) in styrofoam cups with a fragment of one of 16 aquatic plant species. Feeding by the larvae was negligible on 10 species, and only hydrilla, coontail (Ceratophyllumsp.), and naiad (Najassp.) showed heavy feeding damage. However, larvae feeding on coontail had such high mortality (24% per day) that they probably would not do extensive damage to this plant in the field. Since these moths show a relatively narrow host range, the insect could be considered for importation into the United States.