Acute infection in C3H mice produced by the Brazil and Tulahuen strains of Trypanosoma cruzi resulted in intense parasitlzation of the pancreas in both strains, of the adrenals with the Brazil strain, and in parasitization of the skeletal muscle and liver with the Tulahuen strain. The extraordinary intensity of pancreatic and adrenal parasitization is noteworthy, though limited to acute infections. The Brazil strain produced approximately 11 times more circulating trypanosomes in mice than the Tulahuen strain. The number of trypanosomes of either strain required to produce a lethal Infection in C3H mice resulted merely in chronic infection in Swiss mice, with slight parasitization of all tissues except skeletal muscle. This chronic infection was equivalent to that produced in C3H mice by less than half the number of trypanosomes.