The occurrence and geochemistry of sphalerite in the Keno Hill Pb-Zn-Ag lodes is discussed in detail. The Fe content of the sphalerite ranges from 0.63 to 11.54%, the Cd content from 0.71 to 1.16%, and the Mn content from 0.005 to 0.20%. The principal trace elements are Sb, As, Ag, Cu, Pb, Sn, Si, and Al. The Ge and In contents are low. Precise cell edges for the sphalerite were determined and plotted against the mole % FeS. There is a linear relationship between these 2 quantities. An attempt was made to determine the temperatures of crystallization of sphalerite using the mole % FeS present in 10 representative samples. The conclusion reached is that the method is not applicable to the Keno Hill deposits.