Abstract The amoeba, Naegleria gruberi, changes from the amoeboid form to a free-swimming flagellate form in about two hours after it is transferred to distilled water. The change involves the development of a strong polarity and an alteration in the character of the membrane. The cation content of the medium is a controlling factor. There is evidence that the amoeboid and flagellate forms maintain their ionic equilibria by different means and that the membrane is to some extent polarized in opposite directions in the two forms. It is suggested that the flagellate form has an inwardly directed cation pump. Deoxycorticosterone affects the two forms differently and favours the conversion of the flagellate to the amoeboid form. It enhances the slight preference of the flagellate form for K+ rather than Na+. In connexion with the idea that deoxycorticosterone affects the behaviour of the surface of the amoeba, some properties of the packing of molecules into lipoid membranes are discussed. It is shown that the mechanical properties of a surface composed of lecithin and cholesterol differ according to the proportions of the constituents. Such changes could have far-reaching effects on the type of movement of the organism and on the nature of the contacts possible for it.