SUMMARYPycnolejeunea, in recent years, has become an admixture of species belonging to diverse genera:Nipponolejeunea, Tuyamaella, Cheilolejeunea, SiphonolejeuneaandStrepsilejeunea.Its history, delimitation and affinities, at times remote, to these genera, have been discussed. An analysis of the various species of‘Pycnolejeunea’recently monographed by Hoffmann (1935) shows that the large majority of these species do not belong toPycnolejeunea, but demonstrate unmistakable affinities withCheilolejeuneaemend. Schuster (1955); these species have been accordingly transferred toCheilolejeunea, under subgenus nov.Xenolejeunea.BothPycnolejeuneas.str. andCheilolejeuneasubg.Xenolejeuneashow remarkable variation in their stem anatomy and the form and structure of lobule, and this has necessitated their division into several sections; such variation within each genus is discussed.A similar study of the remaining species of‘Pycnolejeunea’sensu Stephani et Hoffmann is de