During an investigation the occurrence of giant muscle fibres was noted in fetal sheep skeletal muscle. The opportunity was, therefore, taken to clarify the histochemical and ultrastructural characteristics of these fibres and to understand if environmental stress such as maternal undernutrition during gestation affects the occurrence of these giant fibres in fetal sheep muscle. Welsh mountain ewes were randomly assigned to either the control group or the nutrient restricted group. The restricted animals received 50% of their daily nutritional requirement from the time of conception until 70 days of gestation and then 100% of their daily nutritional requirement thereafter. The control animals were fed 100% of their daily nutritional requirement for the entire period of gestation. The ewes were killed at 126 1 day of gestation by an intravenous injection of pentobarbitone. For singleton fetuses from each group the semitendinosus muscle was dissected, stained for the examination of histochemical and ultrastructural properties both by light microscope and transmission electron microscope. All singleton fetuses from both control and restricted groups were shown to possess giant fibres in their muscles. The giant fibres were round in shape and larger than adjacent normal fibres. They showed generally strong ATPase-alkaline positive reaction. Regular hexagonal array of myofilaments was absent in the giant fibres. Sarcomer length was shorter than normal fibres. However, there was no significant difference in the number of giant fibres between the control and restricted groups (P>0.05). Maternal undernutrition did not seem to have any effects on the occurrence of giant fibres in fetal sheep muscle. Giant fibres are present in fetal muscle and may result from defects in the developing muscle fibres leading to structural and metabolic anomalies within the fibres.