Restriction enzyme digestion patterns of total genomic DNA revealed the presence of highly repeated DNA in Entamoeba histolytica and E. histolytica-like (Laredo type) amoebae of humans, E. invadens and E. invadens and E. terrapinae of reptiles, and E. moshkovskii isolated from sewage polluted waters. Homology with Plasmodium berghei ribosomal DNA was striking. Northern blot analysis of E. histolytica RNA provided further evidence that the highly repeated DNA codes for ribosomal RNA. The distribution of restriction enzyme sites on repeated DNA of the Entamoeba was highly polymorphic. Thus it was possible, based on EcoRI digestion patterns, to distinguish between strains of E. histolytica and the other species of Entamoeba. Additionally, these investigations at the molecular level corroborate previously reported physiological and biochemical evidence indicating the E. histolytica-like amoebae and E. histolytica are not conspecific. EcoRI fragments from repeated DNA of E. histolytica (strain HM-1: IMSS) were cloned in the plasmid vector pTZ18R. A subfragment from one of these clones proved to be a useful hybridization probe in distinguishing E. histolytica from the other Entamoeba.