Poly-ADP ribose polymerase 1 (PARP-1) is activated by DNA damage and has been implicated in the repair of single-strand breaks (SSBs). Involvement of PARP-1 in other DNA damage responses remains controversial. In this study, we show that PARP-1 is required for replication fork slowing on damaged DNA. Fork progression in PARP-1(-/-) DT40 cells is not slowed down even in the presence of DNA damage induced by the topoisomerase I inhibitor camptothecin (CPT). Mammalian cells treated with a PARP inhibitor or PARP-1-specific small interfering RNAs show similar results. The expression of human PARP-1 restores fork slowing in PARP-1(-/-) DT40 cells. PARP-1 affects SSB repair, homologous recombination (HR), and nonhomologous end joining; there fore, we analyzed the effect of CPT on DT40 clones deficient in these pathways. We. find that fork slowing is correlated with the proficiency of HR-mediated repair. Our data support the presence of a novel checkpoint pathway in which the initiation of HR but not DNA damage delays the fork progression.