This study reports the discovery of a new ebolavirus species, Bundibugyo ebolavirus, which caused a large hemorrhagic fever (HF) outbreak in western Uganda in November 2007. The outbreak resulted in 149 suspected cases and 37 deaths. Initial laboratory investigations using classic methods and a recently developed random-primed pyrosequencing approach identified the virus as a new species distantly related to the Côte d'Ivoire ebolavirus found in western Africa. The genetic divergence of this new virus from all previously recognized ebolaviruses poses significant challenges for diagnostic assays, antivirals, and vaccines. The study highlights the importance of rapid molecular detection and characterization tools in identifying new pathogens and the need for further research to understand the pathogenicity and potential cross-protection of the new virus.This study reports the discovery of a new ebolavirus species, Bundibugyo ebolavirus, which caused a large hemorrhagic fever (HF) outbreak in western Uganda in November 2007. The outbreak resulted in 149 suspected cases and 37 deaths. Initial laboratory investigations using classic methods and a recently developed random-primed pyrosequencing approach identified the virus as a new species distantly related to the Côte d'Ivoire ebolavirus found in western Africa. The genetic divergence of this new virus from all previously recognized ebolaviruses poses significant challenges for diagnostic assays, antivirals, and vaccines. The study highlights the importance of rapid molecular detection and characterization tools in identifying new pathogens and the need for further research to understand the pathogenicity and potential cross-protection of the new virus.