New results of wet-chemical and electron-microprobe analyses are given for an occurrence of johnsomervilleite from the Kiluli pegmatite, Rwanda, and for samples of fillowite from the granitic pegmatite bodies of Buranga and Rusororo, Rwanda, and of Kabira, Uganda, as well as for the closely associated alluaudite. These results and the data collected in the literature are reconsidered on the basis of a crystallochemical formula Na 5 (Ca 2 Na) (sub Sigma 3) X 22 (PO 4 ) 18 . The crystallochemical role of Ca is emphasized. The number of Ca atoms can easily attain 1 among the 22 X cations in this formula. Ca may also occupy all the 8-fold coordinated sites that it normally shares with Na; in such cases, lattice vacancies must occur on the large sites populated by Na. Moreover, the crystallographic parameters of members of the johnsomervilleite-fillowite series, in the hexagonal orientation of the unit cell, clearly increase with the increase in Mn contents. In view of the observed petrographic relationships, johnsomervilleite from Kiluli occurs as a primary phase, associated with an oxidized member of the Li(Fe,Mn)PO 4 series, and sarcopside. In Buranga, Rusororo, and Kabira, fillowite and alluaudite form an actual assemblage; new arguments are provided for an assessment of the non-metasomatic origin of alluaudite occurring in some phosphate associations.