An epizootic in free-ranging lesser flamingos (Phoeniconaias minor) in Kenya resulted in more than 18,500 deaths from August through mid-November 1993. Disease was concentrated along the shores of Rift Valley Lakes Bogoria and Nakuru (Kenya) and did not involve any of the other avian or mammalian species frequenting the lakes. Coincidental to the outbreak was a bloom of algae on Lake Bogoria, toxins from which were first suspected to be causative. Discrete necrotic and granulomatous lesions were often noted in spleen and liver, and Mycobacterium avium serovar I was isolated from both organs. Escherichia coli and Pseudomonas aeruginosa also were often recovered in pure culture from liver. Gross and histopathological evaluation of the cases disclosed signs of acute sepsis and also chronic, potentially life-threatening lesions of mycobacteriosis, primarily involving the spleen and liver. Lesions typical for algae toxicosis were not seen in any birds. Deaths were attributed to septicemia, complicated in those affected, by mycobacteriosis.