To determine the significance of the western black-legged tick, Ixodes pacificus, as a vector of the Lyme disease spirochete, Borrelia burgdorferi, a tick/spirochete survey was conducted in northern California and southwestern Oregon from 1982 to 1984. Of 1,687 adult ticks collected off vegetation, 25 (1.48%) contained spirochetes. Of 715 ticks from Oregon, 14 (1.96%) were infected whereas 11 (1.13%) of 972 ticks from California harbored spirochetes. An isolate of 1 of the spirochetes reacted specifically when treated with monoclonal antibodies to B. burgdorferi. Polyacrylamide gel electrophoresis of a lysate of the isolate showed it to be nearly identical with 2 isolates of B. burgdorferi. Of the 25 infected I. pacificus, 17 had spirochetes in their midgut only; the remaining 8 ticks showed a generalized infection of all the tissues, with midgut, central ganglion and ovary or testes showing heavy spirochetal infections. Decreased immunofluorescent staining reactivity of spirochetes in tissues other than midgut in 6 of 8 I. pacificus with generalized infection may reflect adverse physiologic conditions for the development of spirochetes in the hemocele.