Phallotoxins bind to filamentous actin (F-actin) from liver or rabbit muscle with K _diss∼10⁻⁸ M. By this combination the structure of F-actin is stabilized to such an extent that it will be resistant to the depolymerizing action of 0.6M KI and of deoxyribonuclease I, to denaturation by heat (70 °° C), and to local ruptures caused by ultrasonication or by cytochalasin B. The structural features of phallotoxins essential for affinity to F-actin are specific.