Dynactin is an essential cofactor for the microtubule motor cytoplasmic dynein-1. We report the structure of the 23-subunit dynactin complex by cryo-electronmicroscopy to 4.0 angstroms. Our reconstruction reveals how dynactin is built around a filament containing eight copies of the actin-related protein Arp1 and one of beta-actin. The filament is capped at each end by distinct protein complexes, and its length is defined by elongated peptides that emerge from the a-helical shoulder domain. A further 8.2 angstrom structure of the complex between dynein, dynactin, and the motility-inducing cargo adaptor Bicaudal-D2 shows how the translational symmetry of the dynein tail matches that of the dynactin filament. The Bicaudal-D2 coiled coil runs between dynein and dynactin to stabilize the mutually dependent interactions between all three components.