During the second half of the 19th century, Montréal like the rest of the country of which it is the metropolis, has undergone deep changes more or less related to its industrialization. Within the framework of this article we have considered the year 1871 and examined the following elements: the city and its ethnic groups, the occupation of urban space by social classes and industries. This analysis is based on the computerized data of the 1871 nominal census for 63 blocks in the 9 Montréal districts.