Serum samples from 295 employees of Great Smoky Mountains National Park (GRSM), Rocky Mountain National Park (ROMO), and Grand Teton National Park with adjacent Bridger-Teton National Forest (GRTE-BTNF) were subjected to serological analysis for mosquito-borne bunyaviruses. The sera were analyzed for neutralizing antibodies against six orthobunyaviruses: La Crosse virus (LACV), Jamestown Canyon virus (JCV), snowshoe hare virus (SSHV), California encephalitis virus, and Trivittatus virus (TVTV) belonging to the California serogroup and Cache Valley virus (CVV) belonging to the Bunyamwera serogroup. Sera were also tested for immunoglobulin (Ig) G antibodies against LACV and JCV by enzyme-linked immunosorbent assay (ELISA). The proportion of employees with neutralizing antibodies to any California serogroup bunyavirus was similar in all three sites, with the prevalence ranging from 28% to 36%. The study demonstrated a seroprevalence of 3% to CVV across the three parks. However, proportions of persons with antibodies to specific viruses differed between parks. Participants residing in the eastern regions had a higher seroprevalence to LACV, with 24% (18/75) GRSM employees being seropositive. In contrast, SSHV seroprevalence was limited to employees from the western sites, with 1.7% (1/60) ROMO and 3.8% (6/160) GRTE-BTNF employees being positive. Seroprevalence to JCV was noted in employees from all sites at rates of 6.7% in GRSM, 21.7% in ROMO, and 15.6% in GRTE-BTNF. One employee each from ROMO (1.7%) and GRTE-BTNF (1.9%) were positive for TVTV. This study also has illustrated the greater sensitivity and specificity of plaque reduction neutralization test compared to IgG ELISA in conducting serosurveys for LACV and JCV.