Xenotime and monazite are uncommonly abundant in Precambrian biotite gneiss and migmatite at three localities near Central City, Gilpin County, Colo. The occurrences are in the lower part of a thick layer of migmatized biotite gneiss in a sequence of rocks that have been metamorphosed to the almandine amphibolite facies. The zones of concentration are a maximum of about 5 feet thick and a few hundred feet long, and contain about 1 to 5 percent by volume combined xenotime and monazite.The rare-earth minerals occur dominantly as aggregates of sand-size crystals in thin layers and clots of biotite, which are much coarser than the mica in the typical biotite gneiss. Xenotime is more abundant than monazite in 2 of the 3 occurrences. Both minerals are subrounded to rounded and crystal faces are rare. The two minerals appear to have crystallized contemporaneously. Except for magnetite, other accessory minerals that are common to the country rock are not concentrated with the xenotime and monazite.The field and laboratory data are consistent with the hypothesis that the rare-earth minerals were concentrated at their present sites during migmatization of the biotite gneiss country rock, in a period of Precambrian plastic deformation. Presumably, granitic fluids derived during the deformation selectively mobilized rare-earth cations and phosphate from the biotite gneiss country rock. These ions crystallized with biotite and locally with magnetite to form zones of xenotime and monazite concentrations in migmatized parts of the gneiss.