The systematics ofNybelinia sp. is briefly presented. The percentage incidence and intensity of infection are seen to be high during colder months, and in the smallest and the largest fish. Both sexes are more or less equally susceptible to the infection. The histopathological change in the infected tissue is in the form of a fibrous connective tissue reaction to encapsulate the parasite. A curious mechanism seems to be involved in the formation of ‘Nybelinia cysts’ on the outer wall of the oesophagus. This aspect is discussed in detail.