The insulin-like growth factor type 2 receptor, also known as the cation independent mannose-6-phosphate receptor (Igf2r) is an imprinted gene, which is repressed on the paternally inherited allele in midgestation mouse embryos. We have used a LacZ reporter gene, targeted into the endogenous gene, to investigate the developmental timing, tissue specificity and stability of the repression of the paternal allele. The LacZ expression pattern in pre- and post-implantation embryos, confirmed that Igf2r shows monoallelic expression only after implantation. We show here, additionally, that Igf2r shows biallelic expression in all cells of the preimplantation embryo at E4.5, and, that monoallelic expression is clearly present in all tissues at E6.5, in the early post-implantation embryo. Imprinted expression is maintained in later embryonic development and no tissue was observed to escape imprinting up to E13.5 of development. Thus, for Igf2r, the onset of monoallelic expression occurs in all cells during the implantation period and paternal repression is maintained in all tissues of the late developing embryo.