SYNOPSIS.The endogenous stages ofIsospora seriniAragão andIsospora canariaBox are described from experimentally infected canaries,Serinus canariusLinnaeus. Unlike other Coccidia, the first part of theI. serinilife cycle takes place in mono‐nuclear phagocytes. Five asexual generations are described from this cell type; 2 additional asexual generations and the sexual stages take place in the intestinal epithelium.Isospora canaria, on the other hand, has a conventional coccidian life cycle in that all of the endogenous stages are in the epithelium of the small intestine, with 3 asexual generations and the sexual generation described in the duodenal epithelium. The 2 species differ in their position relative to the nucleus of the intestinal epithelial cell.Isospora seriniis usually on the lumenal side of the nucleus whileI. canariais below the nucleus, toward the basement membrane. The prepatent period is 4–5 days forI. canariaand 9–10 days forI. serini.Patency lasts for 11–13 days inI. canariainfections, but duration of oocyst output is more chronic inI. seriniinfections, persisting for as long as 231 days. Both species have a diurnal periodicity of oocyst discharge which occurs in late afternoon and