Aspects of the spatial and temporal patterns of transmission of Crimean-Congo hemorrhagic fever (CCHF) virus were studied in Senegal, West Africa. A country-wide serological survey of domestic anImals indicated that transmission was most intense in the northern dry sahelian zone and least in the southern, more humid guinean zone. Human IgG prevalence, ranging from nearly 20% to < 1 % among 8 sites throughout the region, also was greatest in the north . A fatal human case of CCHF in Rosso Mauritania in 1988 was studied and an accompanying serosurvev of human contacts and domestic animals indicated epidemic transmission during that period. Systematic samples of adult ixodid ticks on domestic animals allowed us to analyze the distribution and relative abundance of potential CCHF virus vectors, demonstrating that Hyalomma spp. predominated in those biotopes where transmission was most intense. A prospective study of CCHF virus infection and tick infestation in sheep exposed a period of epizootic transmlsslon in 1988 that corresponded temporally with increased abundance of adult H. truncatum and H. impeltatum. Four strains of CCHF virus were isolated from pools of these ticks and of Rhipicephalus guilhoni. Our results suggest that CCHF virus is focally endemic throughout the region, although highly variable in time and space, and that the relative abundance of Hyalomma ticks may be the primary determinant of epidemic transmission.