A leaf-mining fly,Hydrellia pakistanaeDeonier; a stem-boring weevil,Bagoussp. and a tuber-boring weevil,Bagoussp. nr.limosus(Gyll.) are stenophagous insects associated withHydrilla verticillatain Pakistan. Under field conditions and in the presence of its host,H. pakistanaedoes not infest any other plant. In the absence of the host and under confinement it does feed on leaves ofPotamogetonspp. It is widespread, fairly destructive, breeds frequently and is heavily parasitised.Bagoussp. is apparently specific but it attacks host tubers only when the water dries up. Adults and larvae ofBagoussp. nr.limosusdamage stems in water but usually remain restricted to the edges of a water-body because they need to pupate in moist soil on the bank.