A new coccidian species (Protozoa: Apicomplexa: Eimeriidae) collected from the red warbler Cardellina rubra (Swainson) is reported from the Nevado de Toluca National Park, Mexico. Isospora cardellinae n. sp. has subspherical oöcysts, measuring on average 26.6 × 25.4 μm, with smooth, bi-layered wall, c.1.3 μm thick. Micropyle, oöcyst residuum, and polar granule are absent. Sporocysts are ovoidal, measuring on average 19.0 × 12.0 µm, with a knob-like Stieda body, a trapezoidal sub-Stieda body and sporocyst residuum composed of scattered spherules of different sizes. Sporozoites are vermiform with one refractile body and a nucleus. This is the fourth description of an isosporoid coccidian infecting a New World warbler.