We have investigated changes in topography and lateral translational mobility of concanavalin A (Con A) receptors on the surface of cultured chick muscle cells during the period of myoblast fusion. A temporal correlation between these phenomena and the alteration in membrane fluidity known to occur during this time period is established. Receptor topography and mobility are studied by means of a resonance energy transfer technique employing pyrene- and FITC-Con A conjugates. All measurements are performed through a microscope on single cells. Our results reveal that during the period of myoblast fusion Con A receptors undergo a dramatic redistribution on the cell surface. Furthermore, our data suggest that the changes in membrane fluidity observed during muscle differentiation serve to modulate the lateral mobility of these receptors.