DNA methylation differences between maternal and paternal alleles of many imprinted genes are inherited from the male and female gametes and subsequently maintained during development. However, the stages of gametogenesis during which methylation imprints are established have not been well defined. In this study, we used bisulfite sequencing to determine the methylation dynamics of the imprinted genes small nuclear ribonucleoprotein N (Snrpn), insulin-like growth factor 2 receptor (Igf2r), mesoderm-specific transcript (Mest; formerly Peg1), paternally expressed gene 3 (Peg3), and H19 fetal liver mRNA (H19). We identified regions in the maternally imprinted genes (Snrpn, Mest, and Peg3) that were unmethylated in sperm but 100% methylated in mature oocytes. Igf2r, which is expressed from the maternal allele, was completely methylated within intronic differentially methylated region 2 in oocytes and unmethylated in sperm. The 5′ region of H19, a paternally imprinted gene, was completely methylated in sperm and unmethylated in oocytes. We examined the methylation status of Snrpn during oocyte growth and maturation. Whereas the DNA of non-growing oocytes was mostly unmethylated, mid-size growing oocytes had a mosaic pattern of allelic methylation, and full acquisition of the methylation imprint was complete by metaphase II. We have identified regions within imprinted genes that show gamete-specific methylation patterns in mature germ cells and demonstrated that maternal methylation imprints on at least one imprinted gene, Snrpn, are established during the postnatal growth phase of oogenesis. Thus, whereas paternal imprints seem to be established early (in diploid gonocytes well before the onset of meiosis), maternal imprints are established late (in growing oocytes that are arrested in the diplotene stage of meiosis). These findings raise the possibility that assisted reproductive technologies that involve in vitro maturation of oocytes may result in developmental abnormalities due to incomplete methylation imprints in immature oocytes.