Britholite (-Y) occurs as large crystals (5-20 cm) in a zoned granite pegmatite at Reiarsdal, Vest-Agder. Textures suggest that britholite and primary inclusions of a Mn-oxide phase reacted with fluids along fractures, to produce (1) fluorapatite+ calcite + thalenite (s. l.) and (2) tombarthite. Microprobe analyses show that thalenite and tombarthite have essentially inherited the HREE-enriched REE pattern of the britholite. Thalenite appears to have equilibrated with britholite, but the heterogeneity of the tombarthite implies extensive disequilibrium and mobility of most elements during the late hydrothermal alteration of the britholite.