The GTPase dynamin, a key player in endocytic membrane fission, interacts with numerous proteins that regulate actin dynamics and generate/sense membrane curvature. To determine the functional relationship between these proteins and dynamin, we have analyzed endocytic intermediates that accumulate in cells that lack dynamin (derived from dynamin 1 and 2 double conditional knockout mice). In these cells, actin-nucleating proteins, actin, and BAR domain proteins accumulate at the base of arrested endocytic clathrin-coated pits, where they support the growth of dynamic long tubular necks. These results, which we show reflect the sequence of events in wild-type cells, demonstrate a concerted action of these proteins prior to, and independent of, dynamin and emphasize similarities between clathrin-mediated endocytosis in yeast and higher eukaryotes. Our data also demonstrate that the relationship between dynamin and actin is intimately connected to dynamin's endocytic role and that dynamin terminates a powerful actin- and BAR protein-dependent tubulating activity.