In order to investigate content and localization of arsenic in natural fluorapatite, a micro-Raman spectroscopic study was carried out on As-rich fluorapatite crystals from the volcanic region of Mt. Calvario (Mount Etna, Italy). The crystals, from both the unaltered lavas and the metasomatized rocks, show a pure fluorapatite composition in the core, but an arsenic content of up to 15 wt.% As2O5 in the rim of the crystals from the altered lavas. [AsO4]3– ⇔ [PO4]3– substitutions are proposed on the basis of chemical analysis performed with Electron Microprobe (EMP), Scanning Electron Microscopy (SEM-EDS), and Raman spectroscopy. Micro-Raman investigations carried out on the same crystals studied by EMP show a large band, anomalous for fluorapatite, near 860 cm–1, where chemical microanalyses revealed high contents of As2O5. The Raman broad band near 860 cm–1 is produced by the vibrational modes of [AsO4]3– tetrahedra. No evidence of arsenate phase crystallization was found, therefore it is proposed that [AsO4]3– groups substitute for [PO4]3– groups in the apatite structure. Through μ-Raman spectroscopy, it has been possible to relate the iron content revealed by chemical analysis to magnetite in the fluorapatite from the unaltered lava, and to hematite inclusions in the inner core of the fluorapatite from metasomatized areas.