A monograph of the Madagascan endemic genus Pogonostoma Klug, 1835, based on a thorough examination of type specimens and more than 6500 specimens is presented. The infrageneric classification of Pogonostoma partially follows the last revision of the genus (Rivalier 1970), but is significantly modified here. The genus is subdivided into 16 subgenera, ten of them are newly established here. One hundred and nine species and eight subspecies are distinguished, including eight new species described here. The introductory part comprises chapters on material and methods, a history of the taxonomy and nomenclature of Pogonostoma, its biology and distribution in Madagascar, history of the research, remarks on phylogeny, and illustrative explanation of morphology using also SEM microphotographs. A dichotomous key to subgenera, and keys to species groups, species and subspecies are given. Each species (and subspecies, respectively) is described in detail and illustrated from type and other material with colour photographs, line drawings, and mostly also photographs of descriptive features.