Chemotactic stimulation of the cellular slime mould Dictyostelium discoideum by cyclic AMP increases the incorporation of 32P into a 210‐kDa polypeptide. This was determined in lysates prepared from stimulated and nonstimulated cells. Earlier results suggesting that the 210‐kDa polypeptide corresponds to myosin heavy chains [7] have now been confirmed.We have investigated the sequence of events following chemotactic stimulation that lead to the change of myosin heavy‐chain phosphorylation. Chemotactic stimulation did not activate myosin heavy chain kinase, but seemed to elicit the transient accumulation of dephosphorylated myosin heavy chains. Thus, the inefficient incorporation of 32P into myosin heavy chains in lysates of control cells seems to be due to the lack of dephosphorylated myosin heavy chains. Using membranes of aggregation‐competent cells. we found that incorporation of 32P into myosin heavy chains was inhibited by 0. 1–1 mM Ca2+. The effect of calcium seems to be mediated by endogeneous calmodulin and was due to the inhibition of myosin kinase activity rather than to the activation of myosin phosphatase. By the addition of calmodulin, the inhibition of myosin heavy‐chain phosphorylation was further enhanced. Folic acid, an attractant of undifferentiated cells, also caused enhanced incorporation of 32P into myosin heavy chains, as determined in cell lysates. The amount of 32P incorporated in response to cyclic AMP into myosin heavy chains increased during differentiation to the aggregation‐competent stage. Only a small fraction of the cell surface receptors had to be activated in order to elicit a maximal reaction.Our results suggest that a phosphorylated form of myosin heavy chains prevails at the onset of the chemotactic response and that dephosphorylation begins within 5–10 s at 23°C.