Information on the acute and early chronic phases of infection with Trypanosoma cruzi in eight rhesus monkeys is reported with particular reference to the pattern of subpatent parasitaemia and serology. Xenodiagnosis was superior to animal inoculation and blood culture in its capacity to detect subpatent parasitaemia. Each monkey showed an individual, relatively constant pattern of this not related to the nature of the inoculum. Levels of indirect fluorescent and indirect haemagglutinating antibodies remained elevated during a year of observations. One monkey died with a megaoesophagus. Another monkey after challenge with a heterologous strain by orbital inoculation failed to develop Romana's sign and showed no change in serum antibody levels or degree of xenopositivity.