A new Isospora sp. from Carduelis tristis (Aves: Fringillidae) from Ontario, Canada.

Isospora gryphoni n. sp. is described from oocysts found in the feces of the American goldfinch, Carduelis tristis, from Guelph, Ontario, Canada. The oocysts are spherical to subspherical, with a double-layered, smooth, colorless oocyst wall, 29.2 x 30.7 microm (25-33 x 28-34; n = 30) with 2-4 rice-grain-shaped polar bodies; no micropyle or residuum. Sporocysts are ovoid, 22.2 x 13.4 microm (15-25 x 12-14.5; n = 30) with a small Stieda body, indistinct substiedal body, and prominent sporocyst residuum. Sporozoites are vermiform, each with a large refractile body at the posterior end. Forty-nine of 52 (94%) wild-caught adult C. tristis and 1 juvenile bird (minimum 12 days old) were excreting oocysts of I. gryphoni. None of 36 nestlings (maximum 10 days old) was excreting oocysts. Fecal encrustations from 7 of 10 used nests were found to contain oocysts of I. gryphoni.