RESUMO
A new coccidian species, Isospora gilvusi n. sp. (Apicomplexa: Eimeriidae) collected from the warbling vireo Vireo gilvus, is reported from Morelia, Michoacán State, Mexico. Sporulated oöcysts of the new species are spherical to subspherical, 27-31 × 27-29 (30.1 × 28.4) µm, with a length/width (L/W) ratio of 1.1; one or two polar granules are present, but micropyle and oöcyst residuum are absent. Sporocyts are ovoid to drop-shaped, 16-17 × 11-12 (17.0 × 11.5) µm, with a L/W ratio of 1.7; Stieda and sub-Stieda bodies are both present, but para-Stieda body is absent; sporocyst residuum diffuse. At the histological study, endogenous stages were observed in the epithelial cells of the duodenum. This is the second species of Isospora recorded infecting a bird of the family Vireonidae in the New World.