Host specificity is very strongly expressed in a great number of fish parasites, especially in the majority of ectoparasites like isopods. While engaged in the length frequency studies of the Indian Herring, Ilisha filigera (Val.) landed by bull trawlers of the New India Fisheries Company Limited at Sassoon Docks, the isopod parasite, Nerocila sp. was seen very commonly in Ilisha indica during the months of December and February. Though this has been already observed by Chidambaran| and Devidas Menon (1945) in Calicut, the case of host specificity among the species of the same genus has not been pointed out so far.