Dryslwyn Castle in southwest Wales was founded by a Welsh lord in the AD 1220s, captured by the English in 1287, and declined from c.1407 until it was abandoned c.1450. In contrast to historical evidence for changes in procurement of meat, previous zooarchaeological work has suggested that throughout these three periods, there was no change in the supply of animals. We have analysed the stable carbon and nitrogen isotope values of 20 cattle and 25 pigs from the Castle to further test this hypothesis. Our results show that there was no detectable change in the source of supply of cattle to the Castle when it passed from Welsh to English control, though differences in isotope ratios compared to cattle from York suggest that such changes might well be detectable. For pigs, there was an increase in carbon isotope ratios in the English period and a reversion to the previous values in the decline period, which is interpreted as reflecting a change in diet of the pigs from woodland to more open environments. Such a change is anticipated locally due to human population pressure in the English period, but could also reflect a change in the region of supply. This paper shows that there is potential for developing isotope analyses as part of the study of medieval husbandry. Copyright © 2011 John Wiley & Sons, Ltd.