Myxamoebae of the cellular slime mold Dicytostelium discoideum aggregate in response to a chemotactic substance identified as adenosine 3':5'-cyclic monophosphate. Upon aggregation cell division is suppressed, the cells become adhesive, and differentiation is initiated. Freeze-fracture studies of myxamoebae were conducted to determine the effect of cyclic AMP and calcium on plasma membrane ultrastructure. The inner surfaces of the plasma membranes exhibited particulate structures whose sizes (43-187 A) and frequency distribution were determined. Cyclic AMP and calcium induced within 2 hr the formation of particles having average diameters 1.7-times and 1.5-times greater, respectively, than those of the vegetative myxamoebae controls. These data suggest that cyclic AMP mobilizes the intracellular calcium which may be effective in changing plasma membrane structure.