A new isosporoid coccidian species (Protozoa: Apicomplexa: Eimeriidae) is reported from the southern house wren Troglodytes musculus, a very well distributed species in South and Central America. Isospora corruirae sp. nov. oocysts are subspherical to ovoidal, 24.1 × 21.4 μm, with smooth, bilayered wall. Micropyle and oocyst residuum are absent, but small spherules and splinter-like granules are frequently present. Sporocysts are ovoidal to piriform, 14.0 × 9.5 μm. Stieda body is prominent knob-like and substieda body is delicate. Sporocyst residuum is composed of scattered fragments of different sizes. Sporozoites are vermiform with posterior refractile bodies, anterior striations and a nucleus. This is the second description of an isosporoid coccidium infecting a New World wren.