Vermeersiekte in sheep, which is characterized clinically by vomition of ruminal contents, often accompanied by stiffness or paralysis, is caused by various plant species of the genus Geigeria. A histopathological study of the skeletal and oesophageal muscles of three experimental and three natural ovine cases revealed lesions in every case. In paraffin sections examined by light microscopy, vacuolation of focal groups of muscle fibres was seen. The sarcoplasm in the vicinity of the vacuoles was hyalinized and single or multiple centrally displaced sarcolemmal nuclei occurred in the vacuoles. A variation in the size of muscle fibres in these foci was seen in the more chronic experimental cases. Small "atrophic" hyalinized fibres with centralization and proliferation of sarcolemmal nuclei were encountered. Electron micrographs revealed that the vacuoles in the sarcoplasm resulted from focal degeneration of myofibrils in otherwise intact muscle fibres. The thick myofilaments disappeared first, causing dissolution of the A-band in affected myofibrils. Shredding of the remaining thin filaments eventually lead to the complete destruction of myofibrils and the appearance in the fibre of irregular areas of fine granular material, containing remnants of myofilaments, Z-band material and swollen vacuolated mitochondria. Due to excessive loss of myofibrils the diameter of some muscle fibres was reduced.