For some time it has been known that the tick Ornithodoros erraticus might be one of the causes of the persistence of African swine fever in the Iberian Peninsula since its introduction in 1960. In the province of Salamanca serological methods have been used to study the relationship between the presence of the tick in different townships and the outbreaks of African swine fever in these townships between 1987 and 1992. The results showed that there was a statistically significant association between the presence of the parasite and the persistence of African swine fever. In townships without O erraticus traditional methods of control are in most cases enough to avoid new outbreaks of the disease, but in areas with O erraticus the traditional methods should be reinforced by other methods for preventing contact between pigs and the parasite.