Western blots and a kinetic enzyme-linked immunosorbent assay (ELISA) were used to characterize and quantify the prevalence of antibodies to Psoroptes sp. mites in elk (Cervus elaphus) from nine herds in North America. Sera from infested (n = 18) and non-infested (n = 22) elk were used to optimize test methodology and to define cut-off values for negative, suspect, and positive samples. Among 357 samples, 35 (9.8%) of the animals were seropositive, 259 (73%) were negative, and 63 (18%) were suspect. Six of nine herds (67%) contained positive animals and two additional herds (22%) had suspect animals. Sex was not associated with prevalence of antibodies, but adults greater than 2 yr old were approximately five times more likely (95% confidence interval = 2.6-15.4) to be seropositive than calves. Based on these results, we propose that exposure to Psoroptes sp. mites may be widespread in free-ranging elk of North America.