The nucleosome assembly protein Nap1 has been implicated in various cellular functions such as histone shuttling into the nucleus, nucleosome assembly, chromatin remodelling, transcriptional control and cell-cycle regulation in Saccharomyces cerevisiae. In Schizosaccharomyces pombe nap1 null mutant cells are viable but they showed a delay in the onset of mitosis which is rescued by the absence of the replication Cds1 checkpoint kinase. In contrast, the absence of the DNA-damage Chk1 checkpoint kinase is unable to rescue the delay. Moreover, the double nap1 cds1 mutant cells lose viability and cells show positive H2AX phosphorylation, suggesting that the viability of nap1-deleted cells is due to the Cds1 kinase. We also show that overexpression of Nap1 protein blocks the cell cycle in G1 phase.