Abstract. Twenty-two lactating Holstein cattle in Tennessee had clinical signs of intoxication with preformed Clostridium botulinum toxin. These signs included weakness, paralysis of the tongue and chest muscles, abdominal breathing, and, in 11 of the 22 cows, death. Differential diagnoses included hypocalcemia, hypomagnesemia, carbohydrate overload, and several toxicoses including mycotoxin, lead, nitrate, organophosphate, atropine or atropine-like alkaloid, and botulism. A diagnosis of botulism by the ingestion of preformed C. botulinum type B toxin was made by eliminating these other diseases, by finding C. botulinum type B spores in 3 bales of round bale barley haylage fed to these cattle, and by isolating preformed type B toxin from 1 of the 3 bales. Confirmation of the toxin type was made by demonstrating mouse lethality by intraperitoneal injection of specimen extracts with neutralization by C. botulinum type B antitoxin. The haylage, harvested green and encased in black plastic bags to facilitate fermentation, was presumably contaminated by the botulinum toxin when fermentation failed to produce enough acid to lower the pH to 4.5, the pH below which C. botulinum growth is inhibited. Farmers and ranchers who use round hay balers to produce haylage should be alert to this potential problem.Botulism is caused by a neurotoxin produced by Clostridium botulinum that can affect all mammals, fish, and birds. 22 The organism is a spore-forming, gram-positive anaerobic rod that elaborates a potent toxin. 4,23 Eight types of this exotoxin including A, B, C1, C2, D, E, F, and G have been identified. 26 Each type is unique in its geographic distribution and species susceptibility. Only types B, C, and D have been shown to produce disease in cattle. 6 The toxin acts at cholinergic nerve endings, 20 preventing the release of the neurotransmitter acetylcholine, which is necessary for muscle contraction, often causing death by respiratory paralysis. 23 Clinical signs are variable, but often include weakness, paralysis of the tongue resulting in an inability to swallow, paralysis of the chest muscles, abdominal-type respiration, and death. 4 Intoxication occurs by 3 routes: ingestion of the preformed toxin, wound botulism, and intestinal toxicoinfectious botulism. 5 Ingestion of preformed toxin in decaying foods (animal or plant) is the most common form of exposure in humans and adult animals. 15,23 Type B organisms are common in the soil in the midAtlantic states. 22 Two forms of type B toxin occur, a proteolytic toxin, which has maximal toxicity, and a nonproteolytic toxin, which must be activated by trypsin to be fully activated. The proteolytic type B toxin occurs most commonly in the soil of the mid-Atlantic states and Kentucky. 22