We investigated whether host-seeking nymphs and adults of the western blacklegged tick, Ixodes pacificus Cooley & Kohls, the primary vector of Lyme disease spirochetes in far-western North America, are infected naturally with relapsing-fever group spirochetes in Mendocino County, California. Relapsing-fever group borreliae were detected in four (1.7%) of 234 nymphal and two (0.7%) of 282 adult host-seeking I. pacificus ticks by polymerase chain reaction and sequence analysis of the 16S rRNA and flagellin genes, respectively, exhibiting 99 and 98.5% sequence homology to Borrelia miyamotoi Fukunaga. Phylogenetic analysis based on these two genes revealed that the borreliae detected in these ticks belong to the relapsing-fever group and that these are closely related to, if not identical with, B. miyamotoi.