In August 1982, the first autochthonous case of human American trypanosomiasis in California occurred. The isolate, the Tuolumne strain of Trypanosoma, was infective to young laboratory mice and capable of causing death or chronic disease in these animals. The morphology and mensural characteristics are described. This strain can develop in 2 species of Triatominae native to California, Triatoma protracta and T. rubida. The flagellates isolated from these insects were infective to vertebrate hosts. Rhodnius prolixus, a neotropical species, was not susceptible to infection.