Abstract: Dyneins are microtubule-based motor proteins that power ciliary beating, transport intracellular cargos, and help to construct the mitotic spindle. Evolved from ring-shaped hexameric AAA ATPases, dynein's large size and complexity have posed challenges for understanding its structure and mechanism. Here, we present a 6 A crystal structure of a functional dimer of two ~300 kD motor domains of yeast cytoplasmic dynein. The structure reveals an unusual asymmetric arrangement of ATPase domains in the ring-shaped motor domain, the manner in which the mechanical element interacts with the ATPase ring, and an unexpected interaction between two coiled coils that create a base for the microtubule binding domain. The arrangement of these elements provides clues as to how ATP-driven conformational changes might be transmitted across the motor domain.