We present here the results of a large-scale diachronic palaeodietary (carbon and nitrogen isotopic measurements of bone collagen) study of humans and animals from a single site, the city of York (U.K.), dating from the Roman period to the early 19th century. The human sample comprises 313 burials from the cemeteries of Trentholme Drive and Blossom Street (Roman), Belle Vue House (Anglo-Saxon), Fishergate (High and Later Medieval), and All Saints, Pavement (Later and Post-Medieval). In addition, 145 samples of mammal, fish and bird bone from the sites of Tanner Row and Fishergate were analyzed. The isotope data suggest dietary variation between all archaeological periods, although the most significant change was the introduction of significant quantities of marine foods in the Medieval periods. These are first evident in the diet of a small group of individuals from the High Medieval cemetery at Fishergate, although they were consumed almost universally in the following periods. The human isotope values are also remarkable due to unusually elevated δ15N ratios that are not sufficiently explained by the comparably small enrichment in 13C that accompanies them. We discuss the possible reasons behind this and the archaeological significance of the data set. Am J Phys Anthropol, 2007. © 2007 Wiley-Liss, Inc.