An ectoparasitic isopod, Norileca triangulata was found in the branchial cavity of Sardinella gibbosa at Parangipettai coastal waters. The present findings represent the first record of N. triangulata and herein reported. Until now, this species was distributed from Tanimdao Island, Philippines and from Queensland-Eel Reef, Cape York; Michaelmas Cay, near Cairns and Mooloobah, south-eastern Queensland. The range is here extended and now includes to the Southeast coast of India. The materials examined were deposited at the Annamalai University, India (collection Ravichandran). The parasites has been found on 16 out of 16 specimens of S. gibbosa. The prevalence of N. triangulata on S. gibbosa was 7.5 % and mean intensity was 1. The host fish length ranges from 140 to 182 mm. It is further confirmed that the parasites were specific in the selection of host S. gibbosa. Previously N. triangulata was reported from two hosts Parexocoetus brachypterus. Females of N. triangulata ranges 12-18 mm but not found in males. As summarized comparative characteristic feature of two species of parasitic isopods of Norileca indica and N. triangulata. Host species were captured on pelagic region from the coast of Parangipettai. N. triangulata can be distinguished from N. indica by several characters. A related species N. indica has the head to the anterior, and the abdomen facing outwards, pressed against the gill operculum, positioned ventrally in the gill cavity.
Keywords: Cymothoidae; Marine fishes; Norileca triangulata; Sardinella gibbosa.