The effect of cyclic adenosine 3', 5' monophosphate (cyclic AMP ) on ionic movement across the cell-membrane of the cellular slime mold Dictyostelium discoideum, strains V-12 and NC4, has been studied by using radioisotopes, Na²² and Ca⁴⁵. D. discoideum was grown on water agar associated with autoclaved radioactive E. coli 281.The experimental amoebae were centrifuged to free them from bacteria. Cyclic AMP was added to the experimental culture every 10 or 20 minutes. The results showed that cyclic AMP has a significant effect on calcium ion movement across the membrane both in the pre-aggregative amoeboid stage and during the aggregating phase. The movement of sodium ion was changed only during aggregation. The addition of phosphodiesterase the enzyme -which inactives cyclic AMP by converting it to 5' AMP, showed no effect on ion transport either in the pre-aggregative stage or the aggregating stage. The ionic contents of sodium, potassium, and calcium, were measured at the slug stage. The relationship of calcium ion and cyclic AMP to amoeboid movement was compared with the role of cyclic AMP in muscular contraction and other function.