Polymorphic assemblies of F-actin were studied at acid pH using an electron microscope. Three distinct types of ordered aggregates, designated as TYPE I, II and III were found and their basic structural features were discribed. TYPE I was a net with 2-fold rotational symmetry and the tetragon had arms of 320 Å in length and the angles between the arms were 28° and 152°, respectively. For the formation of TYPE I, the optimal concentration of KCl and ATP were found to be 0.1–0.2M and 0.4 mM at pH 5.0, respectively. TYPE II was also a net similar to TYPE I, but the distribution of matter was different from TYPE I and TYPE II was more rigid in structure. The conditions of formation of TYPE II were not elucidated. TYPE III was a side-by-side aggregate of F-actin similar to that formed in the presence of MgCl2 (Hanson, 1967), but TYPE III appeared to be somewhat different in shape. ATP or KCI was not necessary for the formation of TYPE III. F-actin showed ATPase (EC 3.6.1.3) activity at acid pH, like under sonication. This ATPase action was discussed in relation to the formation of the TYPE I paracrystal.