Summary and Conclusions 1. During the campaign in Okinawa, the incidence of infection with Isospora hominis as determined in hospitalized patients exceeded 0.75 per cent. 2. In five patients studied intensively, no symptoms could be ascribed to the Isospora infection, although in some of these the cysts were present in large numbers. The stools were usually negative to the guaiac test and there was evidence that eosinophilia was associated with the usually concomitant ancylostomiasis rather than with the isospora. 3. The infection seemed to be self-limited, although we have observed it to persist for as long as 38 days despite therapy. 4. There is not a dramatic response to treatment with tetrachlorethylene carbarsone, or quinine, and infection can probably take place and continue during atabrine prophylaxis. 5. At a temperature of 27–31°C. mature sporocysts were observed to develop from unsegmented oocysts within 48 hours. 6. Unsegmented oocysts were demonstrated in the upper jejunum in material obtained by Miller-Abbott tube, but no intracellular forms were observed. 7. Long persistence of large numbers, and the fact that the oocysts are usually passed in an unsegmented stage, suggest that Isospora hominis is a parasite rather than merely a passenger through the gastro-intestinal tract.