A Trypanosoma cruzi-like strain was isolated from marmoset monkeys originating from Columbia. The frequency of affected monkeys was up to 32.7%. The parasite density was 10-40 trypomastigote forms per mililiter. Monkeys, rats, mice and other small Rodentia were found to be susceptible. Clinical symptoms or histopathological findings could not be acertained with these trypanosomes even after a 2 1/2 year observation period. The strain behaved as an apathogen in experimental animals. The parasitaemia in animals was sparse (1-10 trypanosomes/ml) and without any conspicuous periodic fluctuations. Trypanosomes could be detected microscopically up to 2 1/2 years after infection in the blood of infected animals. With a positive blood finding, the blood passage could be carried out in animals with regularity. The serum from infected reacted positively in Chagas Latex test as well as in Chagas CFT. The trypomastigote forms injected into cultures of human embryonal thyroid gland and in cells of other origin, showed higher or lower multiplication with a marked cytopathogenic effect between the 9th and 11th day post infection, depending on the type of tissue culture. There was a cyclical development in Rhodnius prolixus or Triatoma infestans. The so-called metacyclic or trypomastigote forms from the faeces of Reduviidae or from tissue culture could again infect animals.