Cofilin/actin-depolymerizing factor is an essential and conserved modulator of actin dynamics. Cofilin binds to actin in either monomeric or filamentous form, severs and depolymerizes actin filaments, and speeds up their treadmilling. A high turnover rate of F-actin in actin-based motility seems driven largely by cofilin-mediated acceleration of directional subunit release, but little by fragmentation of the filaments. On the other hand, the filament-severing function of cofilin seems relevant for the healthy growth of cells. In this study, we have characterized three mutants of porcine cofilin to elucidate the molecular mechanism that underlies the filament-severing activity of cofilin. The first mutant could neither associate with actin filaments nor sever them, whereas it effectively accelerated their treadmilling and directional subunit release. The second mutant bound to actin filaments, but failed to sever them and to interfere with phalloidin binding to the filament. The third mutant could associate with actin filaments and sever them, although with a very reduced efficacy. Of these mutant proteins, only the last one was able to rescue Deltacof1 yeast cells and to induce thick actin bundles in mammalian cells upon overexpression. Therefore, the actin-severing activity of cofilin is an essential element in its vital function and suggested to be exerted by co-operation of at least three distinct sites of cofilin.