Isospora tiaris n. sp. is described from the feces of an adult bird, a sooty grassquit (Tiaris fuliginosa) from Venezuela. The oocysts are subspherical, 27.1 x 23.8 (24.7-30.0 x 21.2-26.5) microns with a bilayered, smooth, colorless wall. A micropyle and oocyst residuum are absent; an ellipsoidal polar granule is usually present. Sporocysts are ovoidal, 14.7 x 10.8 (12.4-16.8 x 8.8-12.4) microns with prominent Stieda and substieda bodies and a residuum composed of small uniform granules.