Determinations of Se contents have been made by X-ray spectrographic means on pyrite, pyrrhotite, pentlandite, and chalcopyrite from a variety of Canadian ores including a high-intensity magnetite replacement body, several nickeliferous copper sulfide (magmatic) ores of Sudbury, 9 massive, nonnickeliferous, Cu, Fe, (Zn) hydrothermal replacement ores chiefly from Quebec and Ontario, 10 pyritic gold- quartz deposits, banded siderite-pyrite (chert) deposits, Michipicoten, and pyritic, uraniferous ores of Algoma, Ontario. Richest Se concentrations are found in some of the Precambrian nonnickeliferous Cu ores, containing several hundred to 1,000 p. p. m. Se, but even in these, relatively Se-rich and Se-poor provinces are evident. One Precambrian and 2 Paleozoic Cu ores are of the latter type. The Sudbury deposits are intermediate between the 2 extremes, Se in these averaging between 50 and 100 p. p. m. in the 4 common sulfides. Pyrites in gold-quartz ores, in the banded Michipicoten deposits, and in the U ores of Algoma average less than 50 p. p. m. Se but range to below the limit of detectability (15 p. p. m.). In general no order of preference is indicated by the individual sulfides but in several Cu ores and also in the Sudbury ores the tendency is noted for the progressive concentration of Se with the order of crystallization and in some, amounts of pyrrhotite and chalcopyrite are fairly similar. It is suggested that the apparent lack of order in other deposits may be due to contamination of earlier sulfides in a restricted system by late vapors or Cu-rich fluids penetrating earlier minerals. Variations in Se with apparent temperature of formation of pyrite in a Au ore, and vertical and horizontal variations in pyrite and pyrrhotite in several of the different types of deposits are noted in addition to distance from intrusives of diabase and quartz porphyry. Se is somewhat enriched in lower temperature pyrite, a distinct enrichment is noted in the upper levels of more deeply explored deposits, McIntyre and Noranda, and variations in others occur over shorter distances. Horizontal variations with respect to intrusives are irregular to negligible in some cases, but are more regular in others, Se being highest at about 100 ft. from quartz porphyries (McIntyre) and near the contact of larger diabase dikes (Noranda and Geco). Very extensive sampling of all deposits is required before a clear picture can be obtained of the distribution of Se in a single ore deposit. The low to negligible Se content of gold-quartz deposits is attributed to Se-poor magmatic sources and/or wall rocks. In marked contrast to the Se-rich U ores of the Colorado plateau are the low-Se siderite ores of Michipicoten, also associated with tuffs, and the (non-tuffaceous) Algoma U ores in conglomerates, for the pyrite of which a hydrothermal rather than purely sedimentary origin is favored.