A new coccidian species (Protozoa: Apicomplexa: Eimeriidae) obtained from masked yellowthroat Geothlypis aequinoctialis (Gmelin) is reported from Brazil. Isospora piacobrai n. sp. has oöcysts which are subspheroidal to ovoidal, measure 15.8 x 10.5 microm and have a smooth, bilayered wall c. 1.2 microm thick. A micropyle and oöcyst residuum are absent, but a polar granule is present. Its sporocysts are ovoidal and measure 15.8 x 10.5 microm. The Stieda body is knob-like and prominent, the substieda body large, trapezoidal and homogeneous, and the sporocyst residuum composed of granules of different sizes. Sporozoites are vermiform with one refractile body and a nucleus. This is the first description of an isosporoid coccidian infecting a bird of the family Parulidae.