Myxobacteria use soluble and cell-contact signals during their starvation-induced formation of fruiting bodies. These signals coordinate developmental gene expression with the cell movements that build fruiting bodies. Early in development, the quorum-sensing A-signal in Myxococcus xanthus helps to assess starvation and induce the first stage of aggregation. Later, the morphogenetic C-signal helps to pattern cell movement and shape the fruiting body. C-signal is a 17-kDa cell surface protein that signals by contact between the ends of two cells. The number of C-signal molecules per cell rises 100-fold from the beginning of fruiting body development to the end, when spores are formed. Traveling waves, streams, and sporulation have increasing thresholds for C-signal activity, and this progression ensures that spores form inside fruiting bodies.