In Dictyostelium discoideum a number of separate amebae aggregate into a cell mass which later differentiates into a sorus and a stalk. A systematic study was made to see to what extent the cell mass was an equipotential in the sense of Dreisch. Although signs of the beginning of differentiation could be detected earlier than had hitherto been suspected, nevertheless extensive equipotentiality exists and it is possible to convert presumptive spore cells into stalk cells and vice versa at all the early stages.