The first Cl isotopic determinations on biotite from the Ultramafic series and from the J-M reef of the Lower Banded series of the Stillwater Complex, Montana, have δ37Cl values (normalized to seawater, 95% confidence limit error is 0.2‰) ranging from 0.27‰ to −0.93‰; the average is −0.18‰. Assuming minimal fractionation on incorporation into biotite, these data imply an ultimate crustal source for the Cl. However, field, petrographic, and isotopic evidence suggests that high-Cl fluids did not infiltrate from the country rocks after emplacement of the Stillwater magma, nor was the Cl-isotopic signature the result of assimilation of Cl-rich crustal rocks. Stillwater parent magmas have boninitic affinities and can be modeled as second-stage melts from a metasomatically enriched, depleted mantle. We propose that the early melt-extraction event depleted the mantle in F and Cl. Subsequent metasomatism by a Cl-rich agent with a crustal Cl isotopic signature, probably a fluid, produced a source rock with a high Cl/F ratio. In modern magmas, high Cl/F weight ratios are associated with high overall volatile contents, suggesting that the Stillwater Complex and other high-Cl layered intrusions crystallized from magmas that may have contained in excess of 1 wt% water.