Monitoring wildlife use of existing structures beneath roads is critical to understanding the role these structures play in reducing roads’ fragmenting effects. eDNA is increasingly used to determine the presence of wildlife species in terrestrial ecosystems. We tested a novel application of eDNA sampling in wildlife crossing structures under highways—to compare with camera trapping. We compiled lists of terrestrial mammal species detected by both methods for six structures under I-5 in Siskiyou County and six structures under I-10 in Coachella Valley. We then compared species compositions derived from eDNA and camera trap sampling to a list of terrestrial mammals expected at each site, which was generated using CDFW habitat models. On average, eDNA sampling covered a significantly smaller percentage (9%) of expected mammals than camera trapping (15%, p=0.027). However, the species detected in each area varied by method, with eDNA consistently detecting more small mammal species. Next steps include running similarity indices and comparing eDNA results between habitat and substrate type. The best method for monitoring crossing structures depends on the target species: camera traps are likely better for detecting medium-large mammals, eDNA may be better for detecting small mammals that are more difficult to capture with cameras.