Hay mixed with manure and urine residues at sites where hay has been provided as supplemental winter feed for cattle provide an excellent substrate for the development of immature stable flies, Stomoxys calcitrans (L.). Such sites are primary sources of early summer stable flies in the central United States and no effective measures are currently available to control fly development in them. A single application of granular cyromazine in May provided 97% reduction in the number of adult stable flies emerging from hay feeding sites. Stable fly control did not decline during the 12 wk season. A small decline in control was observed relative to anthomyiid, sarcophagid, and syrphid flies developing in the hay feeding sites. However, none of those flies are considered to be pests and > or = 50% control of those flies was maintained for 65 d after application. Cyromazine offers a safe and affordable option for the control of immature stable flies developing in winter hay feeding sites. Controlling those flies should reduce the estimated $2 billion per year of lost production in U.S. cattle industries attributable to stable flies.