The efficacy of a multivalent Pasteurella haemolytica vaccine (Al, A2, T10) in reducing morbidity and mortality associated with pneumonic pasteurellosis in bighorn sheep (Ovis canadensis) was examined. Fifteen captive bighorns were divided equally into three treatment groups based on vaccination status: control (no vaccination), one dose 10 days prior to challenge, or one or two doses 57 wk prior to challenge. At challenge, each bighorn received about 6.2 × 10⁷ colony forming units of P. haemolytica (biotype T, serotype 10, biogroup 4_−CDS, ribotype E_CO; “Alamosa Canyon” strain) suspension sprayed into the proximal trachea. Vaccination reduced (P = 0.1) mortality in bighorns vaccinated 10 days prior to challenge as compared to controls. Although mortality rates in bighorns vaccinated 57 wk prior to challenge did not differ from controls (P = 0.26), a trend in reduced mortality was apparent. Ranked cumulative postmortem scores based on observed gross lesions and bacteriology did not differ (P ≥ 0.14) between vaccinated animals and control animals. Neutralizing antibody titers to P haemolytica leukotoxin were elevated (P = 0.003) at challenge in bighorns vaccinated 10 days before challenge, and neutralizing titers in bighorns from both vaccinated groups were elevated at death ≤7 days after challenge (P ≤ 0.004). In contrast, agglutinating antibody titers to P haemolytica serotype Al or T10 surface antigens did not differ between vaccinated and unvaccinated bighorns (P ≥ 0.19). Based on these data, we believe that this experimental P. haemolytica vaccine is safe and can stimulate protective immunity from pneumonic pasteurellosis in bighorn sheep. Further evaluation of this vaccine as a tool in preventing and managing pasteurellosis in wild bighorn sheep appears warranted.