The temporal and spatial patterns of adult stable fly, Stomoxys calcitrans (L.), emergence from six sites where large round baled hay had been provided to pasture cattle as winter feed were studied using emergence traps. The substrate at these sites, consisting of waste hay mixed with bovine manure and urine, provided an excellent developmental habitat for immature stable flies. Stable flies were the most frequently collected fly emerging from these sites with a yearly average of 1,581 emerging per square meter. Stable fly emergence from these sites began in early May (235 annual accumulated Day-Degree 10°C [DD10]), peaked in late June and early July (400-900 DD10) and then dropped to very low levels in late July (≈900 DD10). The temporal pattern of stable fly emergence from the hay feeding sites differed from that of adult populations measured with sticky traps. Adult populations increased in the spring before significant emergence from the hay feeding sites was observed, dipped in midsummer soon after the hay feeding sites became nonproductive, and then rebounded in the late summer when emergence from the hay feeding sites was very low. The drop in productivity of the hay feeding sites appeared to be because of endogenous factors associated with decomposition of the substrate rather than temperature or precipitation. Winter hay feeding sites appear to be primary sources of stable flies during the early summer, however, they are not responsible for late summer and fall stable fly populations. Overall, the inner most 2-m annulus of the hay feeding sites was the most productive, however, spatial variation among sites was observed. The sex ratio of emerging flies did not differ from 1:1 and the temporal pattern of emergence was similar for males and females. Although several other species of flies were collected emerging from the hay feeding site substrate, house flies (Musca domestica L.) were notably absent.