Most Canadian occurrences of mastodons are from southern Ontario. About four-fifths of them have been found below Lake Warren shore, thus being younger than 12 400 years B.P.; the youngest radiocarbon date is 8 910 ± 150 years B.P. Though most mastodons entered Ontario after the retreat of the Wisconsin ice sheet, a few occurrences may belong to the Mid- and Early Wisconsin interstadials. Association of spruce pollen with mastodon bones and concentration of mastodons in the poorly drained lacustrine plains during the late-glacial and early postglacial time suggest that mastodons preferred spruce forests or woodlands. The extinction of mastodons might have been initiated by gradual shrinking of these spruce forests, and completed by their disappearance from southwestern Ontario, owing to increasing warmth and dryness of postglacial climate, and improvement of drainage along the lowered Great Lakes. Mastodons did not find their way to the northern boreal spruce forests, being separated from them by a wide belt of pine and hardwood forests, which meanwhile had developed over the better drained morainic areas of southern Ontario. Weakened by less suitable food, mastodons became more sensitive to diseases and an easier prey to the Paleoindians.