In a population of spotted hyenas (Crocuta crocuta) monitored between 1996 and 2005 in the Ngorongoro Crater, Tanzania, 16 individuals from five of eight social groups displayed clinical signs of an infection, including severe unilateral swelling of the head followed by abscess formation at the mandibular angle, respiratory distress, mild ataxia, and lethargy. Two (12.5%) of these 16 individuals died within days of developing signs. Clinical signs in hyenas were first noted in 2001, and most cases occurred between September 2002 and February 2003, suggesting an outbreak of infection during this period. Histopathological examination of internal organs from one hyena that died with signs revealed morphological changes consistent with severe bacterial infection. Phenotypic examination and phylogenetic analysis of the 16S rRNA gene of the causative agent of infection revealed a Lancefield group C Streptococcus with a high level of homology to S. equi subsp. ruminatorum, a subspecies of S. equi recently described in domestic sheep (Ovis aries) and goats (Capra hircus) with mastitis in Spain. Strains similar to this bacterium were also isolated from two hyenas without obvious clinical signs, suggesting that hyenas may be 'carriers' of this bacterium, and from a sympatric Burchell's zebra (Equus burchelli), a herbivore species often consumed by hyenas. To our knowledge this is the first report of a Streptococcus infection in these two wildlife species. The high genetic similarity between the hyena and zebra isolates indicates that inter-specific transmission may occur, possibly when hyenas consume infected zebra carcasses.