The members of the genus Abudefduf (=Glyphidodon, Glyphisodon) - Sergeant majors - of sub-family Pomacentrinae (Fam: Pomacentridae) are primarily tropical marine fishes, generally found along coastal areas of tropical and temperate regions of the world. Their main habitat in the marine ecosystem is rocky pools or coral reefs. These fishes are lively and strongly territorial, subsisting mainly on algae and small invertebrates. Abudefduf sordidus (Forskal), commonly called the ‘Greybanded Sergeant Major’, is a small to moderate-sized species, known to be an exclusive marine dweller, very common and widely distributed in Indo-Pacific Oceans including the Red Sea and Hawaii. Its main habitat being rocky pools and reefs, this species is seldom known to make forays to estuarine brackish waters in the coastline areas. Day (1875-1878) has recorded this species - as Glyphidodon sordidus (Forskal) from the Sind (Pakistan) coasts of Indian Ocean, indicating its wide distribution, but nothing as regards its estuarine visitations. As part of the faunal explorations of the mangrove ecosystem associated with the Kottapuzha estuary at Kolavipalam in Kozhikode District, Kerala, one specimen of A. sordidus (Forskal) was obtained, on 5 October 2001, from the mangrovelined bay waters. The specimen, 14.5cm long (Fig. 1), was collected by a fisherman in his cast net, while fishing at an interior site in the estuary bay, about 0.5km away from the coast-line waters. The local community fishermen remarked that the incidence of this fish species entering the estuarine waters has never been observed earlier, though the specimens of the same or resembling types are known to frequent the rocky-reef environs found along coastal waters and, occasionally, caught during fishing operations. The collection of A. sordidus (Forskal) from the mangrove-lined estuary bay is, therefore, reported here since no such information is so far known pertaining to its foraging visits to estuarine waters in any part of the considerable stretches of estuarine/bra ckish water-zones found along the coasts of Kerala.