A serological survey was carried out in the Central American state of El Salvador to determine the distribution and prevalence of Anaplasma marginale, Babesia bigemina and Babesia bovis. the results suggest that the parasites are widely distributed throughout the country and prevalence rates of 78.5% for A. marginale, 70.5% for B. bigemina and 73.5% for B. bovis were obtained. Tick counts suggest that boophilus microplus is the main blood parasite vector. Epidemiological studies showed that initial infection with the 3 blood parasites occurs in most animals during the first 7 months of life. Notwithstanding these facts which indicate an area of enzootic stability outbreaks of clinical disease were frequently reported. Twenty-two such outbreaks were investigated and management factors predisposing to outbreaks of clinical disease are discussed.