Four 4-5-y-old cows out of a group of 20 developed a toxic myopathy approximately 10 d after being started on a ration contaminated with 21.5% Senna occidentalis beans. Clinical signs included progressive muscle weakness, incoordination of hindlimbs, reluctance to move, dragging of the tip of the hooves of the hindlimbs, and sternal and lateral recumbency. Gross lesions included white to whitish-yellow discolored areas of several groups of skeletal muscle in various regions of the body, but more markedly in the hindlimbs. Significant histopathological changes were restricted to skeletal muscles with variable degrees of segmental degenerative myopathy associated with reparative events. The earliest change seen at electron microscopy of affected skeletal muscles was mitochondrial swelling. More advanced changes ranged from disruption of sarcoplasm and myofibrils to complete lysis of a myofiber segment and early regeneration. Myocardial fibers had swollen mitochondria, disruption of cristae and dense matrical globules. The epidemiology, clinical data and gross, histopathological, and ultrastructural features of S occidentalis poisoning in cattle are presented.