Class differentials in infant mortality were first established for England and Wales following analysis of answers given to the 1911 census. While estimates of these differentials have been made for earlier periods using indirect methods, for the first time this article provides class-specific infant mortality rates based on births and infant deaths in a large English town. Using information provided in Ipswich’s smallpox vaccination registers for the period 1871–1909, a class gradient in infant mortality is shown to have developed in the town from the early 1880s onwards, particularly in the post-neonatal period. Aspects of the class differentials are then examined, including specific causes of death, housing and childcare practices.