The rare-earth elements (REE) in twenty nine calc-silicate minerals, mostly of metamorphic or hydrothermal origin, and catapleiite have been analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). Using pure mineral separates, very low detection limits for REE in minerals (3 to 54 ppb) are achieved, and REE patterns for some minerals with low abundances of REE are reported for the first time. The chondrite-normalized REE patterns of the calc-silicate minerals vary significantly. Although external geochemical factors are important in determining the REE characteristics of metamorphic and hydrothermal calc-silicate minerals, crystal-chemical effects are resolved also. A consideration of stereochemical environments of the Ca site(s) allows the formulation of qualitative rules for selectivity of REE and site occupancy in calc-silicate minerals. The selectivity of REE is controlled largely by the size of the Ca position: in calc-silicate minerals with multiple Ca sites, bond-valence calculation is a powerful first-approximation technique for predicting the site preference of REE.