A field test using diethylstilbestrol was conducted on the 13,000-acre Havens area on Fort Lewis Mountain, Roanoke County, Virginia; reference areas were also selected. Foxes were pre-conditioned to bait acceptance using molded beef suet. A total of 614 suet baits each containing diethylstilbestrol (50 mg) and oral marker (35 mg of demethylchlortetracycline) were distributed seven times from late January to late March. After a 3-week delay, animals were trapped on all areas and examined for reproductive success and marker evidence. Foxes consumed at least 20 percent of all baits and other identifiable species consumed 12 percent; however, unidentified visitors consumed 43 percent. Twenty-one gray foxes (11 marked) and four red foxes (one marked) were trapped on the treated area. Two female red foxes from the treated.area and one from a reference area successfully reproduced. None were marked. All (six) female gray foxes from the treated area were barren and anovulatory. Two of five marked females probably were yearlings; the unmarked female was 4 or more years old. Five of six female gray foxes from reference areas successfully reproduced. The barren female was an anovulatory yearling. At the P<0.025 level (l d.f., chi-square = 5.17) unsuccessful reproduction was associated with the treatment. No other species showed evidence of marker or effects of treatment. / Master of Science