A cotton boll rot was observed in northeast Louisiana in July 1995. The initial developing cotton boll, sepals, and peduncle rapidly become necrotic and die. These tissues appear hardened or mummified. The dead boll and peduncle remain attached hanging to the stem for several weeks before dropping. Diseased and healthy cotton bolls, leaves, petioles, flowers, stems sections and roots were aseptically assayed to identify the mycoflora present. Phomopsis sp. was isolated most frequently from diseased cotton bolls and petioles. In pathogenicity tests, using a sterile toothpick inoculation technique, Phomopsis sp. caused symptoms characteristic to the cotton blossom-boll rot previously observed. Phomopsis sp. was reisolated from the disease bolls completing Koch's postulates.