In Dictyostelium development, prestalk cells first differentiate at scattered positions in the aggregate and then sort out, probably by chemotaxis to cAMP. They may regulate their proportions by selective depletion of the stalk cell inducer, DIF-1. Once sorted, prestalk cells form a DIF-1 sink, which can produce gradients of DIF-1 and its metabolites in the slug. Global movements of cells in the slug may be regulated by cAMP signals, as in aggregation. Terminal differentiation of stalk and spore cells requires activation of cAMP-dependent protein kinase, possibly brought about by ammonia depletion. Finally, a technique for insertional mutagenesis promises the ready isolation of developmental genes.