The mode of attachment of the ancyrocephaline monogenean Protoancylodiscoides mcmsourensis to the gills of the long fin catfish Chrysichthys auratus has been investigated with reference to host-parasite interface. Light and scanning electron microscope observations revealed that the vast majority of worms secured themselves between two adjacent gill lamellae at the area opposite to the efferent or afferent side of the giil filament or at the middle area between the two sides, within the interlamellar space or channel. Frequently, the whole parasite was observed to hide behind the gill lamellae. The parasite preferred clinging to mucous cell-poor microhabitats such as proximal sector of the gill lamellae and avoided mucous cell-rich microhabitats such as the distal region of the gill filament and gill lamellae as well as the uppermost layer of interlamellar epithelium. The worm utilized one pair of large dorsal hamuli and another pair of comparatively small ventral hamuli to attach firmly on the host tissue. However, marginal booklets had no role in attachment. The local histopathological impacts of the parasite on the gill tissues comprised compression and even tearing of the covering epithelium of the affected gill lamella, aggregation of blood corpuscles in the vicinity of haptoral attachment and change in the distribution pattern of different cell types of the affected gill lamella. A few number of lamellae became amorphous. The host response to Protoancylodiscoides infestation comprised gill tissue proliferation (hyperplasia), formation of sinus-like blood collection and occasional appearance of lymphocytes and/or mucus-producing cells in the vicinity of haptoral attachment. Ecological significance of the mode of attachment and selection of highly protected or sheltered microhabitats and interaction between infested gill tissues and invading parasites are discussed in detail.