Using archaeological data of two human intestinal parasites from seventeenth- to early twentieth-century contexts, we explore the intersection of biological and cultural variables that shaped the ecology of cities in northeastern North America during the modern period. These parasites are useful because they require a developmental period in the soil, thus providing a link between human activities and changing environments. Prior to the last decades of the eighteenth century, Trichuris eggs dominate the archaeoparasitological assemblage. Around 1800, there is a shift to increasing proportions of Ascaris eggs, which appears to be largely complete by 1850—a period of increasing urbanization in the northeast United States. Both environmental and behavioral factors play a role in this shift and include the relationship between parasite biology and changing microenvironments, attempts to deal with waste, and use of urban spaces. During this period, poorer households would likely have been at greater risk of parasites because of the ways they used yard spaces, their delayed access to sanitary technology, and the changing nature of urban vegetation in densely occupied neighborhoods.