Robert Josephs! and William F. Harrington abstract: The properties of myosin filaments formed by dialysis of solutions of monomeric myosin (in 0.5 m KC1) against 0.1-0.3 m KC1 in the pH range 6.2-8.5 have been investigated. In the pH range 6.2-7.3 three heterogeneous weight classes of filaments are observed which have infinite dilution sedimentation coefficients (sjo. w) f 1100, 330, and 180 S. Electron micro-scope studiesindicate that the 1100S particles are 2-12 (i long, and the lengths of the 180S particles lie in the range 0.4-3.0 n, and bear many topological similari-ties to the synthetic myosin polymers described by Huxley. In the pH range 8-8.5, a single hypersharp polymer peak was observed with sjo. w= 150 S. A length-distribution histogram obtained from electron microscope measurements indicates that about 70% of the particle lengths lies between 5600 and 7500 A