Particle length and the influence of various factors on the particle length of F-actin polymerized in Vitro were investigated in detail by electron microscopy. The length distribution of F-actin was found to be very heterogeneous; e.g. te number- average length (〈L〉n) was abut 1.5μ, the weight-average length (〈L〉w) was about 3.1μ, and the ratio of the lengths, 〈L〉w/〈L〉n, was 2.1, for F-actin polymerized at a concentration of approximately 0.5mg/ml. When the length distribution was cchecked by flow birefringence measurements, experimental and calculated values of the extinction angle agreed with each other, suggesting that the length distribution measured by electron microscopy reflects that in solution. Judging from the exponentiaal distribution of F-actin particle length, G-actin is polymerized in good accordance with the theories of Flory (1957) and of Oosawa's school(kasai and Oosawa, 1969) for linear and helical aggregates. Aat early stages of polymerization, When only short fragments of F- actin were found, a poisson type of length distribution with a peak at around 1—2μ was observed. This is explained by the rapid growth of pre-existent nuclei. In the final state, the length distribution became exponential. From the dependence of 〈l〉n on the G-actin concentration of polymerization, it is concluded that trimer nuclei spontaneously formed during the initial polymerization process of actin. Sonication of preformed F-actin resulted in fragmentation into short particles of 〈L〉n of 0.1–0.2μ, but after sonication they quickly re-associated with each other to reproduce the initial length distribution. As a physiological modifier of the length of F-actin of β- actinin (in shortens both the weight and number average lengths of F-actin, without, however, changing the extent of its heterogeneity. The action of β- actinin was more pronouced in inhibiting the recovery process after sonication than in shortening the length of F-actin polymerized from G-actin in the presence of β-actinin. Myosin aggregate and ATP also affected the length distribution of F-actin to a coniderable extent. When G-actin was polymerized in the presence of myosin aggragate at a very low ionic strength- conditions, wwhere spontaneous polymerization of G-actin alone occurs to a negligible extent, the resultant F-actin had the same length distribution as that of the mysoin aggergate. In 0.1M KCI, myosin aggerate and ATPalso altered the length distribution of preformed F-actin, decreasing the 〈L〉w/〈L〉n value from 2.0 to1.3. Without ATP the 〈L〉w/〈L〉n value did not change, although both 〈L〉nand 〈L〉w value did not change, although both 〈L〉n and 〈L〉w were decreased.