The Alveolinidae of the Ager Formation (Upper Paleocene-Lower Eocene) in the Tremp Basin have been studied systematically. The commonly used genus name Alveolina has been replaced by Fasciolites because the former is a junior synonym of the latter. One new subgenus, Microfasciolites, and one new species, Fasciolites (Microfasciolites) agerensis, have been established. The new species is the intermediate form between F. (M.) subtilis and F. (M.) boscii. Many species described by Hottinger (1962) and Drobne (1977) appear to be identical with other species, so that the picture of the evolution of the family becomes simpler and more easily surveyed.