Granulocytic anaplasmosis (GA), caused by Anaplasma phagocytophilum, is a potentially fatal, emerging rickettsial disease of humans, domestic animals, and wildlife. The purpose of this study was to determine whether sciurids from multiple areas of northern California were infested with ticks or exposed to or infected with A. phagocytophilum using polymerase chain reaction (PCR) and indirect-fluorescent antibody (IFA) serology. Sciurids of nine different tree- and ground-dwelling species were assessed: arboreal squirrels (western and eastern gray squirrels, Sciurus griseus and S. carolinensis, and Douglas squirrels, Tamiasciurus douglasii) but not northern flying squirrels (Glaucomys sabrinus) had greater evidence of exposure and current infection than did semiarboreal or ground dwelling sciurids (California ground squirrels, Spermophilus beecheyi, and chipmunks, Tamias spp.). Western gray squirrels had the most extensive exposure (70.7% seroprevalence and 12.1% PCR prevalence). Positive squirrels were identified in all regions where squirrels were collected. A logistic regression identified being a western gray squirrel (OR = 20.5, P = 2.95 × 10⁻⁸) and from the north coastal region of California (OR = 9.052, P = 1.41 × 10⁻⁶) as having the highest risk of exposure to A. phagocytophilum. Five of nine sciurid species had evidence of infestation with Ixodes pacificus or I. spinipalpis that could vector A. phagocytophilum. Extensive exposure from multiple areas suggests sciurids may be important in the maintenance of GA in California and indicates that studies of reservoir competence of these species are warranted.