The transcription factor G box-binding factor (GBF) is required for the developmental switch between aggregative and postaggregative gene expression, cell-type differentiation, and morphogenesis. We show that constitutive expression of GBF allows ectopic expression of postaggregative genes, but only in response to exogenous cAMP. GBF activation requires the serpentine cAMP receptors required for aggregation, but not the coupled G alpha 2 or the G beta subunit, suggesting a novel signaling pathway. In response to high cAMP, g alpha 2-null cells can bypass the aggregation stage, expressing cell type-specific genes and forming fruiting bodies. Our results demonstrate that the same receptors regulate aggregation and cell-type differentiation, but via distinct pathways depending upon whether the receptor perceives a pulsatile or sustained signal.