In the course of a field-visit to the south of Madagascar (September/ October 1970), Lepilemur mustelinus leucopus (F. Major 1894) was in tensively observed in order to define the ecological characteristics of this nocturnal folivorous species (Fig. 1a ), which is adapted toparticularly arid conditions in this region. Study of one population provided us with an understanding of the social structure, permittingcomparison with other pro simian species. Subsequently, a laboratory investigation was conducted on various samples from a digestive tractcollected in Madagascar. This permitted more detailed examination of the phenomenon of caecotrophy , which had previously beenobserved in the field.