The sliding clamp is an essential component of the replisome required for processivity of DNA synthesis and several other aspects of chromosome metabolism. However, the in vivo dynamics of the clamp are poorly understood. We have used various biochemical and cell biological methods to study the dynamics of clamp association with the replisome in Bacillus subtilis cells. We find that clamps form large assemblies on DNA, called "clamp zones." Loading depends on DnaG primase and is probably driven by Okazaki fragment initiation on the lagging strand. Unloading, which is probably regulated, only occurs after many clamps have accumulated on the DNA. On/off cycling allows chromosomal zones of about 200 accumulated clamps to follow the replisome. Since we also show that clamp zones recruit proteins bearing a clamp-binding sequence to replication foci, the results highlight the clamp as a central organizer in the structure and function of replication foci.