AbstractWe have found that ultrathin Formvar films are easily and reliably made at an air‐water interface by the drop method. By varying the concentration of Formvar in the drop, films of different characteristics can be obtained. Concentrations of 0.25–0.4% in ethylene dichloride produce extremely flat, ultrathin, and stable films that are especially suited for shadowed and negatively stained preparations. Low concentrations (⩽ 0.1%) produce nets consisting of many tiny holes which, after carbon stabilization, are ideal for supporting high‐resolution samples. Above 0.5%, films made by the drop method develop bubbles, and this bubble defect makes them unsuitable for section support.For section support, Formvar films made by the stripping method off mica are far superior to those made off glass. The films are more uniform in surface contour and thickness. They are less readily attacked by alcohols. Consequently, they are more resistant to staining procedures involving organic solvents and continue to be strong and uniform for section