The Javan rhinoceros (Rhinoceros sondaicus) of Ujung Kulon National Park (UKNP) is the crown jewel of Indonesia's rich natural history. The park lies on a peninsula surrounded by coastline and agriculture-dominated landscapes. The invasion of water buffalo (Bubalus bubalis) into the park carries a substantial health risk to the Javan rhinoceros and threatens plans to establish a new population outside of its only current range in UKNP. Hemorrhagic septicemia (HS), known locally as septicemia epizootica and caused by Pasteurella multocida B:2, could thwart Indonesia's efforts to expand the range of the Javan rhinoceros. Because HS was considered eradicated from Banten Province, few preventative programs have been available to farmers. During June 2012–July 2013, biologic samples were collected from 770 water buffalo in 19 villages. Deep nasal swabs (n=85) were taken for bacterial culture and blood samples (n=770) were collected for serologic testing. No animals were positive on culture. The prevalence of antibody to P. multocida in this population was 1.8% (14 of 770 animals). A structured questionnaire was used to gather information about possible risk factors. Husbandry practices associated with presence of antibody in water buffalo included lack of a permanent area to house buffalo at night, low body condition score (=2), high body temperature (≥40 C), a history of clinical signs or sudden death in the previous year, and a grazing system that utilized significant forage inside the park. Antibody was not associated with sex, age, vaccination status, or season. Understanding HS disease dynamics in the buffalo adjacent to UKNP may improve the livelihoods of people and health of endangered rhinoceroses in this ecosystem.