The spread of foot-and-mouth disease during an epidemic in Northumberland in July, August and September 1966 was analysed. Although strong emitters, for example pigs, were not involved, in 18 of the 32 outbreaks spread could be attributed to the airborne route and in another 4, spread by this route was the most likely. Airborne spread was in general between 1 and 8 km but on two occasions as much as 20 km. Other means of spread included movement by animals, people and vehicles and carriage of virus between animals in neighbouring fields. No spread by milk or milk lorries took place. Sheep were involved in 9 farms; on each, lesions older than 48 h were found and it is likely that the sheep were the source of virus for other animals, especially cattle, on the farm and on neighbouring farms. The analysis suggests that control measures such as slaughtering direct contacts, as used in this epidemic, or ring vaccination as well as movement control would be effective in limiting spread.