Isolated, demembranated Unio gill cilia that have been activated and fixed for thin-section electron microscopy in the presence of 2 mM MgSO4 have 87% of their outer dynein arms attached to an adjacent B subfiber. The distribution of attached arms is uniform with respect to doublet position in the cilium. When both 0.1 mM ATP and Mg++ are added to the activation and fixation solutions, the frequency of bridged arms is reduced to 48%. At the same time, the distribution of the attached arms appears to have been systematically modified with respect to doublet position and the active bend plane. Those doublet pairs positioned in the bend plane where interdoublet sliding is minimal retain a greater number of bridged arms than those doublet pairs positioned outside the bend plane where sliding is maximal. These observations imply a functional coupling of the Mg++-induced bridging of the dynein arms and the subsequent binding and hydrolysis of ATP that results in a force-generating cross-bridge cycle.