Myxophagan water beetles from Madagascar are comprehensively revised. Holotypes of the two previously known speciesare re-described and illustrated. Six new species of Incoltorrida Steffan and one new species of Hydroscapha LeConte aredescribed and illustrated. The larvae of Incoltorrida quintacostata are illustrated and discussed. The presence of peritrichciliates of the genus Platycola Kent on specimens of Incoltorrida madagassica Steffan is discussed and illustrated.Habitus, habitats, and male genitalia are illustrated, and distributions are mapped. The following new species are described(type localities parenthetic): Incoltorrida benesculpta n. sp. (Fianarantsoa, 3.2km S Ambohimanjaka); I. galoko n. sp.(Antsiranana, Diana, Ambilobe, Antsaba, Galoko mountains ); I. magna n. sp. (Antsiranana, Diana, Ambilobe, Antsaba,Galoko mountains); I. marojejy n. sp. (Antsiranana, Sava, Marojejy National Park); I. quintacostata n. sp. (Fianarantsoa,3.5km N Ivato); I. zahamena n. sp. (Toamasina, Alaotra-Mangoro, Zahamena National Park); Hydroscapha andringitran. sp. (Fianarantsoa, Ambilavao, Sendrisoa, approx. 10km N of Andringitra National Park).