Brush border myosin I (BBMI) is a single-headed, unconventional myosin from intestinal microvilli, composed of a heavy chain of relative molecular mass 119,000 (M(r) 119K) and three calmodulin light chains. Although believed to have a largely structural role, it exhibits the normal actin-activated ATPase and motility properties of a member of the myosin superfamily. Here we present three-dimensional maps of BBMI-decorated actin filaments with and without bound MgADP. While the motor domain remains in a state similar to rigor, the light-chain-binding domain swings through approximately 32 degrees, resulting in a approximately 50-A movement at the end of the region visualized (the second calmodulin light chain). This could correspond to approximately 72-A movement of the entire domain. Although qualitatively similar to the movement observed in myosin II, the magnitude of the change is sufficiently different to suggest that structural changes during the actomyosin ATPase cycle differ among myosins, possibly reflecting adaptation for specialized functional demands.