Plutarch, Arrian, Diodorus, Justin, and other ancient historians report that rumors of poisoning arose after the death of Alexander in Babylon in 323 B.C. Alexander’s close friends suspected a legendary poison gathered from the River Styx in Arcadia, so corrosive that only the hoof of a horse could contain it. It’s impossible to know the real cause of Alexander’s death, but a recent toxicological discovery may help explain why some ancient observers believed that Alexander was murdered with Styx poison. We propose that the river harbored a killer bacterium that can occur on limestone rock deposits. This paper elaborates on our Poster presentation, Toxicological History Room, XII International Congress of Toxicology, Barcelona, 19-23 July 2010, and Society of Toxicology Annual Meeting, Washington DC, March 2011.