Deposits of massive pyrite and pyrrhotite-pyrite occur mainly in volcanic rocks and particularly in the uppermost pyroclastic parts of submarine volcanic piles. Large deposits of massive pyrite and pyrite-sulfur have formed in the recent past and are forming now around fumaroles and volcanic hot springs in Japan and Taiwan. The older but undeformed massive pyrite deposits of Cyprus appear to have formed in the same manner, but largely under submarine conditions. Deposits in deformed rocks, as those at Rio Tinto, Spain, and Shasta county, California, give less positive evidence of origin but appear to be of the same type. Material in volcanic emanations is thought to be in both a molecularly dispersed and a colloidally dispersed state where emanations enter a near-surface submarine environment. Rapid flocculation of colloids and rapid precipitation of material in solution are postulated to occur in sea water or in sea-water-soaked pyroclastic rocks and sediments. Several mechanisms for formation of dense bodies of massive sulfide are suggested, but filtration of the flocculated and precipitated material, with accompanying replacement, appears to be a probable process.