D.Sc. / A new species of the family Diplozoidae is described from 'the Vaaldam, Transvaal, South Africa. This species Paradiplozoon kimberleyensis n. sp., was initially found on the gills of the large mouth yellow fish Barbus kimberleyensis Gilchrist and Thompson, 1913 and later also on the.gills of the small mouth yellow fish Barbus aeneus (Burchell, 1822). Parasites collected from these fishes were studied with the aid of whole mounts, serial paraffin and methyl acrylate sections and the electron microscope. P. kimberleyensis n. sp. differs from the other African diplozoons with respect to the position of the productive system relative to the region of fusion, the lobular 'structure of the testis and the absence of the egg filament. Reconstructions of the reproductive system and prohaptoral region were used in describing the relevant structures. From these, it was clear that the vas deferens of one individual is fused to the common vitelline duct of the partner in copulation. Furthermore, the relations of the different parts of the reproductive system with respect to each other, could clearly be demonstrated. The physiological reproductive processes (i.e. oogenesis, spermatogenesis and vitellogenesis) of P. kimberleyensis n. sp. follow the basic pattern of other monogenean parasites. The various components of the reproductive system were discussed and compared with those of other diplozoons. Finally, the biological significance of the fusion of two separate individuals into a functional unit and the possible ways in which this phenomenon was accomplished were discussed.