AbstractThe unspecialized haplolepideous peristome, short irregularly incrassate, densely pluripapillose upper leaf cells, strong single costae that have well differentiated guide cells, strongly toothed leaves and short-beaked opercula of the genusMesotuspoint to a closer relationship of this genus with the Dicranaceae than to the Dicnemonaceae. On the basis of a series of unusual character states (capsules immersed, calyptrae mitrate, opercula conic, rhizoids arising from laminal leaf cells, and massive, endosporically developed protonemata)Mesotusis placed in its own subfamily (Mesotoideae, subfam. nov.). Within the DicranaceaeMesotusappears closest toSclerodontiumby virtue of its pluripapillose, irregularly incrassate, upper leaf cells and its prominently bordered leaves. The genus consists of a single species (M. celatus) found throughout New Zealand. One collection ofM. celatushas been attributed to an unspecified Australian locality.