111 out of 113 passerine birds of 13 species were infected with species of Isospora. The variability of oocyst measurements in all 13 species of bird was high indicating the presence of more than one normal distribution. All birds other than jackdaw and hedge sparrow harboured oval and spherical oocysts. The spherical and oval oocysts of greenfinch and sparrow were examined separately and each was found to be markedly heterogeneous. Studies of endogenous stages from greenfinch and sparrow revealed two species: I. lacazei with schizonts separating merozoites without residual cytoplasm, with spindle-shaped microgametes and macrogametes with dense cytoplasm and no obvious plastic granules; I. chloridis n. sp. with schizonts leaving a residuum when separating merozoites, with comma-shaped microgametes and macrogametes with pale cytoplasm and distinct plastic granules. Relating these endogenous studies with the sporulaticn time of 60 - 72 hours, the prepatent period cf 5 days and oocyst morphology it is shown that no additional species were present in greenfinch and sparrow. Thus the heterogeneity in oval and spherical oocyst populations is not explained in terms of species, rather in terms of conditions supplied by host cells. Oocyst output shows a diurnal periodicity, being greatest at 18.00 hours and least between 6.00 and 13.00 hours. Cytochemical studies were carried out for the detection of DNA, RNA, carbohydrates, proteins, lipids and enzyme activity. DNA was detected in all stages except the macrogamete. RNA was present in all stages in the cytoplasm and nucleus. Three types of carbohydrate were present: glycogen, a polysaccharide combined with protein end acid mucopolysaccharide. The first two contribute to the oocyst wall. Special accumulations of protein were found in the oocyst wall and in the refractile globules of sporozoites. Acid phosphatase activity was detected in the nucleus and cytoplasm, alkaline phosphatase only in the nucleus.