For the first time, an attempt has been made to quantify the effect of a copepod parasite (Ergasilus bengalensis) on the respiratory efficiency of the gills of a riverine catfish (Wallago attu) using an important parameter – the diffusing capacity. In addition, scanning electron microscopy has been used to demonstrate the effect of a monogeneid fluke on the gill lamellae of this catfish. The anchorage of E. bengalensis towards the leading edge of the interlamellar channels (pores) will affect laminar flow of water through the gill sieve. About 35% of the water flow (1.23×10−3 cm3/s) and lamellar area (15.41 mm2) of an infected gill filament (4.46 mm) is affected by the copepod parasite (1.59 mm). Other effects of the monogeneid flukes on the catfish included fusion of the tips of gill filaments and distortion of lamellar epithelium. Parasite-induced changes in the branchial organization are discussed in relation to oxygen uptake efficiency of the gills of the catfish.