ICP0 is a multi-functional herpes simplex virus type 1 (HSV-1) immediate-early (IE) gene product that contributes to efficient virus growth and reactivation from latency. Here we show that HSV-1-induced cell-cycle arrest at the G2/M border requires ICP0 and Chk2 kinase and that ICP0 expression by transfection or infection induces ATM-dependent phosphorylation of Chk2 and Cdc25C. Infection of cells with a replication-defective mutant virus deleted for all the regulatory IE genes except ICP0 (TOZ22R) induced G2/M arrest whereas a mutant virus deleted in addition for ICP0 (QOZ22R) failed to do so. Chk2-deficient cells and cells expressing a kinase-deficient Chk2 did not undergo cell-cycle arrest in response to TOZ22R infection. Chk2 deficiency diminished the growth of wild-type HSV-1, but not the growth of an ICP0-deleted recombinant virus. Together, these results are consistent with the interpretation that ICP0 activates a DNA damage response pathway to arrest cells in G2/M phase and promote virus growth.