MOVING cells display a variety of shapes and modes of locomotion1, but it is not clear how motility at the molecular level relates to the locomotion of a whole cell, a problem compounded in studies of cells with complex shapes2-5. A striking feature of fish epidermal keratocyte locomotion is its apparent simplicity6. Here we present a kinematic description of locomotion which is consistent with the semicircular shape and persistent 'gliding'; motion of fish epidermal keratocytes. We propose that extension of the front and retraction of the rear of these cells occurs perpendicularly to the cell edge, and that a graded distribution of extension and retraction rates along the cell margin maintains cell shape and size during locomotion. Evidence for this description is provided by the predicted circumferential motion of lamellar features and the curvature of 'photo-marked' lines within specific molecular components of moving keratocytes. Our description relates the dynamics of molecular assemblies to the movement of a whole cell.