This paper proposes that one function of the open fields was to reduce the transaction costs of cow-keeping by lowering commonersí costs of bulling. At enclosure, cow-keeping fell among small owners who, unlike large farmers, had difficulty obtaining bulling services and were not substantial enough to own both the bull and the cow; they were therefore worse off with enclosures. The minimum acreage required to restore cow keepers to their pre-enclosure economic position indicates that even commoners who were given some land at settlement lost out with the change in property rights.