At the onset of S phase, chromosomal replication is initiated by the loading of DNA polymerase alpha onto replication origins. However, the molecular mechanisms for controlling the initiation are poorly understood. Using Xenopus egg extract, we report here the identification of a Xenopus homolog of Cdc45, a yeast protein essential for the initiation of replication, which is shown to be an essential molecule for the initiation of replication via the loading of DNA polymerase alpha onto chromatin. XCdc45, by physically interacting with the polymerase in the extract, became associated with chromatin only after nuclear formation. During S phase, XCdc45 co-localized with the polymerase in the nuclei, and the loading of the polymerase, which depended on endogenous XCdc45, was facilitated by exogenously added recombinant XCdc45. These findings, together with the apparent requirement of S-phase-cdk activity for the loading of XCdc45, suggest that XCdc45, under the control of S-phase cdk, plays a pivotal role in the loading of DNA polymerase alpha onto chromatin.