Aggregation in the primitive cellular slime mold Dictyostelium minutum proceeds by means of chemotaxis toward a continuously secreted folic acid analog [De Wit, R. J. W. & Konijn, T. M. (1983) Cell Differ. 12, 205-210]. The onset of culmination is marked by the appearance of concentric waves of cell movement on the aggregate surface. Culmination proceeds by the chemotactic attraction of amoebae to the center of wave propagation, which results in the accumulation of amoebae into a finger-like structure. Evidence is presented that the chemoattractant used during culmination is cAMP, which is secreted in pulses. The cells secrete cAMP themselves; cAMP receptors and phosphodiesterase activity appear on the cell surface just before the onset of culmination. Micromolar concentrations of externally applied cAMP induce disorientation of amoeboid movement at the onset of culmination. These observations are compatible with the hypothesis that the cAMP signaling system organizes multicellular development in both primitive and advanced cellular slime mold species. Advanced species such as Dictyostelium discoideum use this signaling system also in an earlier stage of development to organize the process of cell aggregation.