Acute cases of contagious caprine pleuropneumonia frow outbreaks in Kenya were investigated. A mycoplasma, designated F38, was isolated in 41 cases from 30 outbreaks but no organisms identical to M,,rcaplasma mycoides subspecies carri represented by the reference strains PG3, Smith and N108, were recovered. Similarly no organisms identical to M. mycoides subspecies mycoides of caprine origin represented by strain Vom were isolated from acute cases. F38 was pathogenic to goats by inoculation and the pleuropneumonia induced spread readily to goats in contact. Primary isolation of this organism proved difficult and growth in vitro was slow. A second mycoplasma, designated F30, was isolated from three chronic cases of caprine pleuropneumonia. F30 was pathogenic to goats and sheep following inoculation but the disease was not contagious. This organism grew readily in vitro even at primary isolation. Serological, biochemical, cultural, and morphological studies suggested that F38 and F30 were members of the species, M. mycoides. Complement fixation and agar gel diffusion test results emphasized the similarity of F30, F38, and reference strains of the M. mycoides species. Growth and metabolic inhibition tests indicated that F30 but not F38 belonged to Al- Aubaidi's Group 8. Moreover F38 differed from M. mycoides subspecies carri strains PG3, Smith, and N108. A small preliminary trial yielded evidence that a degree of protection to the contagious disease was engendered by the prior inoculation of goats with F38 passaged serially in cultures. Strains F30 and F38 were sent to three International Mycoplasma Reference Centres for further study. Reports placed F30 as a member of the M. mycoides subspecies mycoides in Al- Aubaidi's Group 3.. Classification of strain F38 is in progress; if F38 proves to re- present a new serctype, a name will be proposed.