Food is well-known to encode social and cultural values, for example different social groups use different consumption patterns to act as social boundaries. When societies and cultures change, whether through drift, through population replacement or other factors, diet may also alter despite unchanging resource availability within a region. This study investigates the extent to which dietary change coincides with cultural change, to understand the effects of large-scale migrations on the populations' diets. Through stable carbon and nitrogen isotope analysis of Iron Age, Roman, and Early Medieval human bone collagen, we show that in Croatia large-scale cultural change led to significant changes in diet. The isotopic evidence indicates that Iron Age diet consisted of C3 foodstuffs with no isotopic evidence for the consumption of C4 or marine resources. With the Roman conquest, marine resources were added to the diet, although C3 foodstuffs continued to play an important role. In the Early Medieval period, this marine component was lost and varying amounts of C4 foodstuffs, probably millet, were added to the otherwise C3 diet. In both of these transitions it is likely that the changes in diet are related to the arrival of a new people into the area. Am J Phys Anthropol 2012. © 2012 Wiley Periodicals, Inc.