The fission yeast cdc23 gene is required for correct DNA replication: cdc23 mutants show reduced rates of DNA synthesis and become elongated after cell-cycle arrest. We have cloned the Schizosaccharomyces pombe cdc23 gene by complementation of the temperature-sensitive phenotype of cdc23-M36 and confirmed the identity of the gene by integrative mapping. Analysis of the DNA sequence reveals that cdc23 can encode a protein of 593 amino acids (Mr=67 kDa) with 22% overall identity and many structural homologies with the product of the Saccharomyces cerevisiae DNA43 (MCM10) gene which is required for correct initiation of DNA synthesis at chromosomal origins of replication. Construction of a cdc23 null allele has established that the cdc23 gene is essential for viability, with cdc23 deletion mutant spores germinating but undergoing arrest with undivided nuclei in the first or second cell cycle. The S. pombe cdc23 gene on an expression plasmid is able to complement the S. cerevisiae dna43-1 mutant. These structural and functional homologies between two distantly related species suggest that cdc23 and DNA43 may represent genes for a conserved essential eukaryotic DNA replication function.