Five of 16 (31%) rufous-capped warblers (Basileuterus rufifrons) and 2 of 5 (40%) ovenbirds (Seiurus aurocapilla) sampled from Costa Rica were positive for a novel species of Isospora. Oocysts have a thin, smooth, double-layered, colorless wall and measure 22.3 μm ± 1.6 μm × 24.3 μm ± 1.5 μm (19–25 μm × 21–28 μm) with an average length-width (L/W) ratio of 1.0 (1–1.3). Oocyst residuum and micropyle are absent, but 0–4 spherical to cigar-shaped polar granules (1–2.5 μm) are present. Sporocysts are ovoid and measure 11.8 μm ± 0.9 μm × 16 μm ± 1.7 μm (10–14 μm × 12–19 μm) with an average L/W ratio of 1.6 (1.0–1.9). A knob-like Stieda body continuous with the sporocyst wall and a trapezoidal compartmentalized substieda body are present. Each sporocyst contained 4 sporozoites and a diffuse sporocyst residuum consisting of many variable-sized granules, some as large as 2 μm. This is the second description of an Isospora species in New World warblers (Passeriformes: Parulidae) and the first report of Isospora from both the rufous-capped warbler and ovenbird.