Extremadura is an extensive region (41,000 km2) located in the Southwest of Spain, where animal husbandry is a very important economical resource. Livestock parasitism of ticks is common in the region and it is favoured by the environmental conditions and management systems. Mediterranean theileriosis (MT) is perhaps the most important tick-borne disease of cattle. Although Theileria annulata has been identified in Extremadura since 1933, little epidemiological information is available. During the last decade, we have conducted different studies with the aim to clarify the epidemiological situation. Laboratory diagnosis, identification of tick species and studies on their seasonal dynamics have been carried out. Moreover, T. annulata was isolated and cultivated and experimental transmission of the parasite was made. Finally, several seroepidemiological tests in livestock from different origins have been performed. From 1991 to 1995 a total of 236 of MT clinical cases have been diagnosed. The following tick species were identified: Hyalomma marginatum marginatum (64.0%), Rhipicephalus bursa (25.6%), Hyalomma lusitanicum (9.6%) and Rhipicephalus sanguineus (0.5%). Clinical cases are detected from May until November with peaks in May-June and October that correspond to the periods of maximum activity of adults of H. m. marginatum and H. lusitanicum, respectively. From our results of seroepidemiological studies the Extremadura region could be considered as endemic for MT. The disease is unstable in semi-extensive systems (dairy cows) and tends to stabilisation on extensive farms for meat and fighting bulls.