A one-week investigation was performed at Mkunki and Mavumo (ward of Shume, Lushoto district, Tanzania) at the time of outbreak (June, 1980). Rodents, people, domestic dogs and fleas were examined, 36 rats, mostly Rattus rattus, were caught and used in serological and bacteriological analysis for plague. 48 fleas, mostly Xenopsylla brasiliensis, were collected from the captured rodents. 413 people and 11 dogs were examined. All rodent, carnivorous and human sera were tested for plague antibodies, using the Passive Haemagglutination (PHA) test. Similarly protein extracts from rodent organs were tested for Yersinia pestis fraction I. Impression smears prepared from these organs were stained with methylene blue and examined for presence of bipolar bacteria. Bacteriological isolation of Y. pestis was performed on bubo aspirates and/or blood samples of ten patients. Over-all total and infested flea indices of rodents were 1.3 and 3.7, respectively. Bipolar organisms were demonstrated in 13.3% of the rodent smears while specific agglutinating plague antibodies were detected in 3.3% of the tested rodent sera. Y. pestis fraction I antigen was detected in 8.3% of the rodents. Of all the human sera tested, 1.4% contained specific plague antibodies. Y. Pestis was bacteriologically isolated and biochemically confirmed in 16.7% of the cultured specimens. All the dog sera were negative for plague. It was generally concluded that the causative agent of the outbreak was Yersinia pestis and that the disease probably spread from Rongai (Rombo district).