The imprinted H19 gene is hypomethylated on the active maternal allele and hypermethylated on the repressed paternal allele in the somatic tissues of mice and humans. We previously demonstrated that the paternal-specific methylation of a 2 kb region located between -2 and -4 kb relative to the start of transcription is maintained throughout murine development, and we thus propose that this region is crucial to determining the imprinted expression of H19. Here, we test the correlation between differential methylation and imprinted expression by analyzing the mouse H19 gene in the undermethylated extraembryonic tissues. During early and midpostimplantation stages, > 95% of the H19 RNA is derived from the maternal allele. Dissection of yolk sac revealed that the paternal allele is expressed at a low level in the viseral endoderm but is completely repressed in visceral mesoderm. Bisulfite methylation analysis of yolk sac DNA showed that the maternal allele was hypomethylated and that 95% of the paternally derived clones were hypermethylated. Thus in extraembryonic lineages, the majority of H19 DNA is differentially methylated. These results lend further support to the hypothesis that DNA methylation confers the imprint on H19.