Ionophores are antibiotic feed additives to control coccidia and increase feed efficiency in cattle (Bos taurus and B. indicus). Inclusion of an ionophore is nearly ubiquitous in cattle feeds in the United States. Cattle are very tolerant of ionophores, with established LD50s far in excess of the indicated feeding rates. There is no established LD50 for any ionophore in the water buffalo (Bubalus bubalis). There is one case report and a very small dosing trial which showed water buffaloes to die after ingestion of as low as 5mg/kg of monensin. Monensin is the most commonly used ionophore in the United States, sold by Elanco under the trade name Rumensin. Water buffaloes are popular in the developing world as draft animals and for meat and milk production. As modern agricultural practices become more commonplace in the developing world, more opportunity will exist for water buffaloes to be fed cattle feed containing ionophores. It will be important for veterinarians who may work in these areas to be aware of this potential toxin. This report describes a suspected case of monensin toxicosis in a water buffalo from a facility co-housing Holstein dairy cattle and water buffalo, and feeding a diet containing monensin to all animals in the facility