Developing Dictyostelium cells form large aggregation streams that break up into groups of 0.2 × 105 to 1 × 105 cells. Each group then becomes a fruiting body. smlA cells oversecrete an unknown factor that causes aggregation streams to break up into groups of ∼5 × 103 cells and thus form very small fruiting bodies. We have purified the counting factor and find that it behaves as a complex of polypeptides with an effective molecular mass of 450 kD. One of the polypeptides is a 40-kD hydrophilic protein we have named countin. In transformants with a disrupted countin gene, there is no detectable secretion of counting factor, and the aggregation streams do not break up, resulting in huge (up to 2 × 105 cell) fruiting bodies.