Summary (1) A brief description is given of the plague focus at Rongai in Kenya, which extends northwards to the Baringo bush country, is bounded on both sides by the escarpments of the Rift Valley, reaches Njoro in the south, and includes the extinct volcano of Menengai. (2) During an outbreak late in 1952, huts and Rattus burrows were dusted with 3 per cent. gammexane and 10 per cent. DDT dusting powders. Patients were treated with streptomycin. No fleas appeared on ectoparasite-free white rats placed nightly in treated huts for over a month, whereas in untreated huts 25 fleas ( X. cheopis and X. brasiliensis ) were collected. (3) In January, 1952, Arvicanthis abyssinicus was the predominant wild rodent at Rongai, and multimammate mice ( Mastomys coucha ) were also numerous. Other species included Rattus rattus kijabius, Rhabdomys pumilio, Otomys angoniensis , and Taterillus, Dendromus, Lemniscomys, Leggada, Euxerus , and Crocidura species. Seven Aethomys kaiseri were also obtained. During the dry weather in March, 1953, comparatively few wild rodents were found, but the Otomys population had increased. At Keruguya 839 Rattus were caught in villages during July and August. (4) Xenopsylla cheopis and Dinopsyllus lypusus were the dominant rodent fleas in the Rongai Area, and X. brasiliensis was found only in small numbers. X. cheopis occurred in fair numbers on both Rattus and wild rodents. D. lypusus is mainly a field species and was particularly numerous on M. coucha . Ground squirrels ( Euxerus sp.) yielded only Ctenocephalides crataepus , while gerbils ( Taterillus sp.) were chiefly infested with X. nubicus . At Keruguya X. brasiliensis predominated on Rattus , and none was found on wild rodents which were only lightly infested with other species of fleas. (5) At Rongai 10 strains of Pasteurella pestis were isolated from A. abyssinicus , five from M. coucha , two from R. pumilio , and three from O. angoniensis. P. pestis was also isolated from X. cheopis and D. lypusus . At Konza pooled spleens from six live Arvicanthis and one M. coucha proved infected. (6) Susceptibility tests indicated that Arvicanthis and M. coucha from Rongai were mostly highly resistant to P. pestis; some survived inoculation, while others lived from 15 to 38 days. Otomys were only moderately resistant, dying after 4 to 9 days. The most susceptible animals were gerbils ( Taterillus sp.) which invariably succumbed after 3 days. (7) During the second visit to Rongai in 1953, when enzootic conditions prevailed, P. pestis was isolated from wild rodents and fleas but could not be recovered from Rattus; this seems to prove that wild rodents are the primary reservoir of plague at Rongai. The chief animals involved are Arvicanthis, M. coucha and Otomys , which live in close ecological association. There is no evidence that Rattus is the primary reservoir in Kenya, and we believe that this domestic species is always secondarily infected. (8) X. cheopis is regarded as the chief transmitting agent at Rongai, and D. lypusus , which has been shown to harbour P. pestis , may be an important vector of sylvatic plague in Kenya. (9) Fifty-two cultures of P. pestis from man, Rattus , wild rodents and fleas, acidified glycerine, reduced nitrates, but did not affect rhamnose; most of the strains were from Rongai but others came from Konza, Kiambu, and Tanganyika; all showed marked sensitivity to a specific bacteriophage. It would appear, therefore, that the plague bacilli of Kenya and perhaps Tanganyika belong to the ancient group recently designated Pasteurella pestis var. antiqua by Devignat (1952).