Three additional cases of E. polecki infections are reported, bringing the total number found in man to 10. Morphological characteristics of the trophozoites and cysts are described and illustrated, with emphasis on the tendency for one type of nuclear structure to predominate in trophozoites and another in cysts. E. polecki probably is much more common than the literature indicates, probably many cases being diagnosed as E. histolytica on the basis of saline or iodine smears. Positive identification can be made only with iron-hematoxylin stained preparations.