Mckinstryite occurs as late-stage, primary sulfide in the U = Co-Ni-Ag-Bi deposit of the Echo Bay Mine, N.W. Territories, accompanying niccolite, rammelsbergite, chalcopyrite, bornite, covelite and early dolomite. Electron microprobe analyses reveal an composition of Cu (sub 0.72) Ag (sub 1.28) S (sub 0.91) . A variable excess of Ag and S deficiency are probably due to a nonstoichiometric defect structure of the Schottky type. The mineral has an average S.G. of 6.64+ or -0.04, is light gray, weakly bireflectant, strongly anisotropic and biaxial positive. The reflectance at 470nm = 30.8% (av. min), 36.7% (av. max); at 546nm = 29% (av. min), 32.8% (av. max); at 589nm = 23.8% (av. min), 30.7% (av. max); at 650nm = 30.1% (av. min), 29.1% (av. max). VHN 20 gm = 50.1 to 67.1 (av. 55.7). Powder diffraction fives a = 13.962 + or -0.001A, b = 15.675+ or -0.002A, c = 7.775+ or -0.002A and Z = 32, delta S 34 values of +23 to +27 per thousand exhibited by the mckinstryite are interpreted as being due to the late formation of the mineral at 94.4 degrees C.