Nineteen (91%) of 21 rufous-and-white wrens (Thryothorus rufalbus) and five (71%) of seven plain wrens (Cantorchilus modestus) sampled from Costa Rica were positive for a new species of Isospora. Oocysts have a thin, smooth, double, colorless wall and measure 20.1 ± 1.4 × 23.4 ± 1.5 μm (18-24 × 20-26 μm) with an average length-width ratio of 1.2 μm. Sporocysts are ovoid, measure 9.5 ± 0.9 × 15.5 ± 1.1 μm (7-12 × 12-18 μm) with an average length-width ratio of 1.6 μm. A nipple-like steida body continuous with the sporocyst wall and a prominent oval-shaped substeida body are present. In addition to the four sporozoites, a single compact sporocyst residuum was present in each sporocyst. This is the first description of an Isospora species from the family Troglodytidae and the first report of Isospora from the rufous-and-white wren and plain wren.