West Bengal, situated in the eastern part of the country, is one of the most beautiful and important states of India. In a developing country like India, fish is one of the cheapest sources of protein and West Bengal contributes a major portion of the total fish production. Fishery development represents a vital sector in West Bengal. The degree of success in this sector depends to a great extent on the advancement of fish parasitology research, as it is evident that the increase of fish yields can only be achieved from healthy fish stock. During an investigation (July 2001 - December 2002), a protozoan ectoparasite, Chilodonella hexasticha (Kiernik, 1909) Kahl, 1931, was found on the gills of a freshwater fish Nandus nandus (Hamilton). This finding is very important for the fishery sector, as this parasite causes the disease chilodonellosis, which leads to mass mortality. The present paper deals with the morphology, systematics, seasonal incidences and prevalence of C. hexasticha.