1. The behaviour of the adult Amoeba proteus has been studied in their aquarium culture media, in distilled water, and in dilute pure solutions of various acids and bases of known and graded pHs.
2. Amoebae belonging to three physiological types are described from the cultures. The majority are large, active, digitate forms, which adhere closely to the substratum, and are grey in colour when seen in transmitted light. On transference to pure media they become attenuated, floating forms, to which the name type ‘A’ Amoebae has here been applied. The remainder are darker in appearance and less adherent in the culture media. They belong to two types, which from their behaviour in pure media have been separated as type ‘R’ and type ‘C’ Amoebae.
3. The association between the floating form and the capacity for adhering to the substratum is stressed.
4. The problems of adhesion, of attenuation in pseudopodia, and of the relationship between the types of Amoebae are discussed.