A new species of Trypanosoma, T. (Megatrypanum) hoarei, is described from shrews of the genus Sorex (S. trowbridgii, S. vagrans, S. ornatus, and S. palustris) in California. Four body types are found in adult populations of T. hoarei, two common and two rare, as well as intermediate stages, indicating a tendency to-ward polymorphism in this species. Multiplication is discontinuous, with a reproductive and growth period characterized by moderate numbers of diverse and bizarre developmental forms in the blood and high coefficients of variation for total length (20–30 %), succeeded by an adult phase with few trypanosomes in the blood and lower coefficients of variation for total length (7–12 %). The method and site of multiplication in T. hoarei is unknown but evidence indicates that reproduction by multiple fission or binary fission of amastigote forms may occur in tissue capillaries, with the production of large, rounded amastigote individuals. The latter develop, without intervention of epimastigote stages, into large, rounded trypomastigote individuals from which a wide variety of other developmental forms proceed, including certain distinctive, very long ribbon-like trypanosomes. A review of the literature on mole and shrew trypanosomes suggests that T. hoarei of Sorex, T. petrodromi from Petrodrorrms, and T. talpae from Talpa are related, primitive insectivore forms, clearly distinguishable from the lewisi-type trypanosomes also found in insectivores.