MYOSIN II crossbridges interact with F-actin producing power-strokes of around 100 Angstrom (refs 1, 2), during which the products of ATP hydrolysis are released(3,5). This has been postulated to involve an articulation of the myosin head (S1) on actin, or substantial conformational changes in S1 itself(6-8). Small movements of the regulatory light chain have been detected (see, for example, refs 9, 10), but most data suggest that the bulk of S1 does not move on actin during crossbridge cyclings(8,11). Here we present three-dimensional maps of S1-decorated F-actin in the presence and absence of MgADP. The myosin motor domain is similar in both states but there are major orientational differences in the chain-binding domain, This domain acts as a rigid level arm pivoting about the end of the motor domain and swinging similar to 23 degrees, resulting in a similar to 35-Angstrom step. Small, nucleotide-mediated conformational changes in the motor domain(14-16) may thus be converted by the light-chain domain into large movement steps.