Four bacterial developmental systems are described. In the dimorphic cell cycle of Caulobacter crescentus , differ- AU:1 ences in the proteins assembled at cell poles cause cell division to generate a stalked cell rich in regulator DivK-P, and competent for continued proliferation, and a swarmer cell rich in regulator CtrA-P, and unable to proliferate until it discards its flagellum and grows a stalk. The other three systems all lead to the formation of spores, but by completely different routes. In Bacillus subtilis , an endospore forms inside a mother cell; in the mycelial Streptomyces coelicolor , long hyphae grow into the air and then turn into chains of spores; whereas in Myxococcus xanthus , which hunts in motile swarms to prey on other bacteria, the swarm aggregates into a mound to form a fruiting body, inside which cells change into spores. The regulatory cascades leading to differentiation evolved completely independently in the four systems, but show some common strategies.