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The kinetics of actin polymerization were analyzed by taking into account nucleation, elongation, and spontaneous fragmentation of filaments. Polymerization curves measured in the presence of potassium (40 mM) were found to be in good agreement with curves calculated for the assumption that nucleation and elongation but no fragmentation reactions occur. Polymerization curves measured in the presence of calcium (1.8 mM) or magnesium (0.6 mM MgCl2 and 0.5 mM EGTA) could only be stimulated by calculated curves when spontaneous fragmentation was assumed to occur in addition to nucleation and elongation. The experiments reported in this study that even in the absence of ultrasonication or shear forces actin filaments may break spontaneously and that the extent of fragmentation depends strongly on the experimental conditions. Spontaneous fragmentation changes the shape of the polymerization curves significantly. When fragmentation of filaments takes place, a relatively long lag phase of polymerization is observed that is followed by a strongly increasing polymerization rate to reach the final constant value quickly. On the other hand, when filaments are formed exclusively by nucleation, the polymerization curves approach the final constant value slowly after a relatively short initial lag phase.