The homotrimeric DNA replication protein proliferating cell nuclear antigen (PCNA) is regulated by both ubiquitylation and sumoylation. We study the appearance and the impact of these modifications on chromosomal replication in frog egg extracts. Xenopus laevis PCNA is modified on lysine 164 by sumoylation, monoubiquitylation, and diubiquitylation. Sumoylation and monoubiquitylation occur during the replication of undamaged DNA, whereas diubiquitylation occurs specifically in response to DNA damage. When lysine 164 modification is prevented, replication fork movement through undamaged DNA slows down and DNA polymerase delta fails to associate with replicating chromatin. When sumoylation alone is prevented, replication occurs normally and neither monoubiquitylation nor sumoylation are required for the replication of simple single-strand DNA templates. Our findings expand the repertoire of functions for PCNA ubiquitylation and sumoylation by elucidating a role for these modifications during the replication of undamaged DNA. Furthermore, they suggest that PCNA monoubiquitylation serves as a molecular gas pedal that controls the speed of replisome movement during S phase.