Summary 1. Twenty-six species of bacteria (41 strains) were tested with regard to their ability to cause excystation and growth of E. histolytica in vitro. A heterogeneous group of 14 species of bacteria were found capable of doing so when each was cultured with sterile cysts. The bacteria having the property of inducing excystation and multiplication were Leptotrichia buccalis, Clostridium perfringens, Staphylococcus aureus, Salmonella schottmuelleri, Streptococcus hemolyticus, Str. faecalis, Str. viridans, Bacillus subtilis, B. mesentericus, Actinomyces muris, Aplanobacter stewartii, Bacterium coronofaciens, Serratia marcescens and Neisseria catarrhalis. Some of the intestinal bacteria such as Alcaligenes faecalis could not initiate growth of amoebae from cysts. Three species of yeasts also failed to induce excystation. 2. The growth relationships of the amoebae to the bacteria were determined. During the most active period of multiplication of the amoebae the bacteria were sharply reduced in number and were ingested by the amoebae. 3. The effect of a mixed bacterial flora was studied. Growth was greatly stimulated in some cases when a second or third species was added to an amoeba culture already growing with one species of bacterium. 4. Encystation was produced when Cl. perfringens, a bacterium uniformly present in the human colon, was added to cultures containing one or two other species of bacteria. Cultures containing Cl. perfringens, Str. hemolyticus and Lepto. buccalis or Cl. perfringens and Str. hemolyticus were particularly good cyst producers. Such cultures were uniformly rich in trophozoites but the cysts varied from 5 to 40 per cent. 5. It is felt that the bacteria play a significant role in the excystation, multiplication and encystation of E. histolytica and studies of the bacterial action may lead to a knowledge of the conditions necessary to cultivate E. histolytica without bacteria.