Striated-muscle contraction is propelled by the concerted action of many myosin II molecules assembled into highly organized, semi-crystalline bipolar thick filaments (TFs). Myosin II spontaneously polymerizes into synthetic filaments upon lowering the ionic strength thereby providing a structural-functional model, yet the exact backbone structure and the mechanisms whereby the length and diameter of vertebrate TFs are regulated, are still little understood. Here we investigated the topographical structure of TFs by using high-resolution AFM under aqueous buffer conditions at various ionic strengths. Synthetic TFs were obtained by dialyzing rabbit longissimus dorsi myosin II against buffers containing 0-120 mM KCl. Filaments were deposited on freshly-cleaved mica and imaged to obtain qualitative and quantitative structural parameters: morphology, length, width, and height. Surface-adsorbed TFs, with lengths ranging between 800-4500 nm, appeared as axially bipolar structures flattened by surface stabilization, thereby exposing the underlying subfilamentous features. TFs were splayed towards their ends, indicating that the interactions holding together the individual myosin molecules are progressively weaker towards the N-terminal head domain. Myosin molecules often emerged from intertwined subfilaments, the number of which ranged between 3-10 per cross section. A compact core structure was frequently present, occupying approximately 25 % of the filament length. Conceivably, the compact core serves as the site of linkage between the C-terminal ends of the myosin II tail domains. Filament width, indirect measure of structural compactness, was 102.66±23.38, 177.71±56.30, 160.06±41.53, 154.93±45.85 and 145.04±38.57 nm at 0, 30, 60, 90 and 120 mM KCl, respectively, indicating that TFs are structurally loosened even by small concentrations of ions, but the interactions via the myosin tails are strong enough to maintain overall structural integrity. The electrostatically modulated TF compaction may contribute towards structural dynamics necessary for interactions with various sarcomeric proteins.