A new locality of the rare Brazilian liverwort Myriocoleopsis puiggarii (Lejeuneaceae) the only species in the genus, was discovered in the river Pardo near Barra do Turvo, Sao Paulo State. Data on stem-anatomy, oil-bodies, sporophyte and ecology are provided here for the first time. The species grows on rocks and trunks of shrubs in or near running water. Sporophytes are developed only in plants growing on emerged substrates. Myriocoleopsis seems to be most closely related to Cololejeunea subg. Chlorolejeunea (type; C. madothecoides). The stem in Myriocoleopsis, consisting of c. 30 cortical cells surrounding 1 large medullary cell, is more complex than in all species of Cololejeunea except C. madothecoides. Striking variation in the stem-anatomy of the latter species was observed.