The role of the western black-legged tick (Ixodes pacificus) versus that of other potential arthropod vectors in the epidemiology of Lyme disease was evaluated by determining the prevalence of anti-arthropod saliva antibodies (AASA) among residents (n = 104) of a community at high-risk (CHR). Salivary gland extracts prepared from I. pacificus, the Pacific Coast tick (Dermacentor occidentalis), the western cone-nose bug (Triatoma protracta), and the western tree-hole mosquito (Aedes sierrensis) were used as antigens in an ELISA. Sera from 50 residents of the San Francisco Bay region in northern California and 51 residents of Imperial County in southern California served as comparison groups. The prevalence of AASA ranged from 2% for A. sierrensis to 79% for I. pacificus in study subjects, 0% for D. occidentalis to 36% for I. pacificus among residents of the San Francisco Bay region, and 6% for I. pacificus to 24% for A. sierrensis in residents of Imperial County. The associations between AASA and demographic factors, potential risk factors, probable Lyme disease, and seropositivity for Borrelia burgdorferi were assessed for 85 members of the CHR. Seropositivity for I. pacificus and B. burgdorferi were significantly correlated, the relative risk of seropositivity to B. burgdorferi was about 5 (31% versus 6%) for subjects who were seroreactive to I. pacificus, nearly every individual who was seropositive for B. burgdorferi had elevated levels of antibodies to I. pacificus, and the mean titer for antibodies to I. pacificus was significantly higher for subjects seropositive versus those seronegative for B. burgdorferi. Together, these findings support the widely held belief that I. pacificus is the primary vector of B. burgdorferi for humans in northern California, and they demonstrate the utility of the AASA method as an epidemiologic tool for studying emerging tick-borne infections.