Microinjected phalloidin induces both time and concentration-dependent changes in morphology and motility of amoebae and acellular slime moulds. In A. proteus injection of a 10(-3)M solution of the drug causes a separation of cortical hyaline plasma from central granular plasma. Simultaneously protoplasmic streaming and cellular locomotion are lost irreversibly. Lowering the concentration of phalloidin to 2 x 10(-4)M results in a reversible disturbance; amoebae recover after 30 to 60 minutes and show normal movement. In Ph. polycephalum the injection of a 10(-3)M solution of phalloidin into single veins induces a local gelation of the protoplasm followed by the separation of hyalo- and granuloplasm. In semi-thin and ultrathin sections the hyaline plasma regions contain a fine granular groundplasm rich in ribosomes but free of cellular organelles. The central granular plasma consists mainly of membrane-surrounded cellular compartments. The two morphologically distinct plasma regions are separated by a 0.5 to 1.0 micrometer layer of filamentous material. In A. proteus the filamentous layer is found shortly after phalloidin injection in close proximity to the plasma membrane, and consists of thin 5 to 6 nm filaments. With increasing time this layer contracts, separates from the inner plasma membrane and moves to the interior of the cell. During contraction thicker filaments with diameters of 10 to 30 nm and lengths of 300 to 500 nm are formed. The results indicate that the display and contraction of the phalloidin-induced filament layer can account for the changes observed in cellular movement and cytoplasmic organization. The resulting phenomena i.e. separation of hyaline plasma from granular plasma and changes in both the protoplasmic streaming pattern and locomotory activity of the cells, are discussed in terms of a general understanding of amoeboid movement.