Ureaplasmas were isolated from the oropharynx of all of 22 male and 19 female adult marmosets and from the genital tract of about a quarter of them. These ureaplasmas seem to be natural inhabitants of the oropharynx and not of human origin because half the animals had had very limited human contact and preliminary serological tests indicate that the organisms are not the same as the known human serotypes. 11 babies born in captivity were found to have oropharyngeal organisms usually within 24 h of birth and the oropharynx of the parents was thought to be the most likely source. The marmoset may be a useful model for studying the role of ureaplasmas in human disease.