1. Closely packed cells ofDictyostelium discoideum, grown in a suspension culture, form aggregation patterns whose specific structures reflect the functional states of the cells. 2. Prior to the development of these structures, waves of excitation appear which propagate with a velocity of about 40μ/min. If functioning rhythmically as a whole, the wave-propagating center gives rise to a system of separate, concentric waves. If, on the contrary, parts of the center become active successively, a continuous spiralshaped excitation front is formed. 3. It is proposed, that the propagated waves associated with an autorhythmic center inhibit the formation of similar centers in its vicinity ("spacing effect"). 4. The aggregates formed by the closely packed cells are often too large to give rise to a single sorocarp. In this case a mechanism becomes active which divides the cell mass; each of the resulting parts transforms itself into a sorocarp.