The spatial pattern in Senegal of Crimean-Congo hemorrhagic fever (CCHF) virus IgG antibody prevalence in human and sheep was determined as was the relative abundance of potential tick vectors. A systematic, country-wide serological survey of sheep demonstrated that 10.4% of sheep exhibited IgG to CCHF virus. Sexes were infected equally. Antibody prevalence increased with age from 2.1% during the first year to 18.2% among sheep greater than or equal to 3 years of age. IgG prevalence was highest in the northern, arid Sahelian zone, averaging 75.7% seropositivity, and decreased to zero in the southern, moister Sudano-Guinean and Guinean zones. Human IgG prevalence ranged from 21% to less than 1% among the 8 sites that were sampled throughout the country, being greatest in the arid north and least in the south. Hyalomma ssp. ticks predominated in those biotopes where antibody prevalence was highest. The abundance of Hyalomma ticks may be the proximal determinant of endemic transmission.