This study analyzes the demographic characteristics of a previously neglected area in colonial America—the urban center. Growth, birth, and death rates in Philadelphia between 1720 and 1775 are estimated using a variety of sources. Immigration, smallpox, economic vacillations, and a skewed age structure are attributed primary responsibility in determining the level of and changes in Philadelphia's vital rates. The elevated level of these rates is evident in a comparison with vital rates in Andover and Boston, Massachusetts, and Nottingham, England.