Comparative topography and ultrastructure was done for the tegument architecture between a polyopisthocotylean parasite (Metamicrocotyla cephalus) and a monopisthocotylean parasite (Dactylogyrus extensus) for the first time. The result showed that the tegument of M. cephalus is characterized by many foldings producing closely packed annular corrugations superimposed with a complex configuration of many robust papillae bearing microvillus-like projections, thin terminal web just underneath the apical plasma membrane, membranous projections from the basal plasma membrane inside the syncytium, one type of tegumental cell producing two tegumental secretory bodies inside the syncytium (electron-lucent bodies "ts1" and electron-dense bodies "ts2") and the syncytium is thick (l.3-3 microm). The tegument of D. extensus was characterized by the weak annulations and many minute folds like microvilli, some invaginations in the apical plasma membrane form small vacuoles, eruption of some secretory bodies through the apical plasma membrane, three kinds of secretory bodies inside the syncytium from two different tegumental cells (small electron-dense granules "td1" and large fusiform moderately electron-dense bodies containing fibrous threads "td2" from one cell type and thin dumbbell-shaped electron-dense bodies "td3" from other cell type) and the syncytium is thin (1.4-1.5 microm). The possible functions of the distinctive features of the tegument whether presence or absence have been discussed, especially the proposed role played during the life of the parasite and the possibility of making them as characteristic features of the polyopisthocotyleans than the monopisthocotyleans.