The Middle Iron Age on Öland (around 200-550 AD) is often regarded as a prosperous period witha wealth based on animal husbandry. In this study charred cereals from several Iron Age sites atÖland are studied to answer questions about prehistoric diet and agricultural practices. Themethod used is stable isotope analysis of carbon and nitrogen in the cereals, and one further aim ofthe study is to evaluate this method. The results suggest that there is little need for pre-treatment ofcereals before isotope analysis. Most of the grains analyzed were hulled barley and in all sites thereare indications of intensive manuring, as would be expected in permanent field agriculture. Thering forts of the period may here have been places where an agricultural surplus was gathered.Concerning human diet, the isotope values indicate cereals may have been an important part.Crops may also have been used to feed the livestock, possibly with secondary products like straws,and likely to a different extent in different animal species. Finally, the sites from the Middle IronAge all appears to have been abandoned. Heavy dependence on animal manure may havedecreased the resilience of agriculture, making it more vulnerable to unexpected changes, forexample the climate downturn after 536 AD.