Summary Isosporosis in man is due to infection with Isospora belli and I. hominis, of which the former is the more prevalent form. Differential diagnosis is easily made on discovering the immature oöcysts of I. belli and the mature sporocysts of I. hominis in freshly evacuated stools. Until 1935 there were only 200 known cases of human isosporosis in the world. At present at least 835 authentic cases have been diagnosed in the Americas, including isolated infections from many localities and endemic foci in São Paulo, Cali and Santiago. Examination of thousands of human stools, employing satisfactory concentration technics in addition to direct fecal smears, has indicated that the percentage rates of isosporosis in these three endemic centers are approximately 0.1, 1.0 and 1.8, respectively. Although human isosporosis may become chronic, much more frequently it is only a temporary unstable infection in man and the dog. The one plausible explanation which we are able to suggest for the maintenance of I. belli and I. hominis endemically in a community is based on the resistance of the cysts to unfavorable environmental conditions, thus affording ample opportunity for human exposure. Greater competence in the recognition of the stages of Isospora in the feces, use of a satisfactory concentration technic, and examination of thousands of fecal specimens are required to provide an accurate picture of isosporosis in a community.