In 1819-20, measles swept through the native populations of the fur trading lands of the Canadian Northwest. This was the first such epidemic in this region and was accompanied by mortalities characteristic of virgin soil epidemics. This study seeks to establish the origins of this epidemic and the pathways by which it reached the Northwest. It then examines in detail the diffusion of the disease throughout the Petit Nord, or the eastern section of the Northwest lying to the east of Lake Winnipeg and between Hudson Bay and Lake Superior. Finally, it attempts to reconstruct the distribution and magnitude of the mortalities among the native peoples of this region. The evidence indicates that the measles diffused from the northeastern U.S. from an endemic focus comprised of the cities of Baltimore, New York, and Philadelphia. The virus was then relocated westward along two routes, entering the Canadian Northwest at Brandon House, on the eastern plains, and at Fort William, at the western end of Lake Superior. Within the Petit Nord, the disease was confined to the more heavily populated souihwestern sector, where it was carried by North West Company canoe brigades from Fort William. Among the afflicted populations, the severity of the disease and resultant mortality varied widely, with as many as two-thirds succumbing in some places and relatively few at others. The 1819-20 measles epidemic marked the beginning of a new era in the disease history of the Canadian Northwest. lt was the result of the breakdown in the isolation of the Northwest, a process which accelerated as the nineteenth century progressed and led to frequent epidemics of diseases previously absent from the region.