Differential epigenetic modification by methylation of CpG dinucleotides is a candidate mechanism that may identify the alleles of imprinted genes and result in monoallelic expression of either the maternal or the paternal allele. Determination of the allelic methylation status of imprinted genes in the gametes and during early development is constrained by the limiting quantities of genomic DNA available from these early developmental stages. To circumvent this problem we have used bisulfite genomic sequencing to determine the allelic methylation status of the minimal promoter and a 1-kb region within theXistgene during preimplantation development. We find that the parentalXistalleles are not differentially methylated in these regions. Our findings are discussed in the context of previous conflicting data obtained using methylation-sensitive restriction enzyme digestion followed by PCR amplification to assay for methylation.