Meiosis, the fundamental evolutionarily conserved differentiative process by which haploid gametes are produced, is a complex and tightly regulated nuclear process. The murine Meig1 gene was previously shown to have a germ cell-specific transcript which is abundantly expressed during meiosis, in both males and females, suggesting that it is involved in meiotic processes. Protein analysis revealed that MEIG1 appears in multiple phosphorylated forms, including two dimeric forms of M(r) 31,000 and 32,000, which exhibit a developmentally regulated switch in their relative abundance. The tyrosine-phosphorylated M(r) 31,000 form becomes the dominant form once the cells enter meiosis. In this study we show that the M(r) 31,000 dimeric form appears in the nuclear fraction of testicular protein extract, whereas the M(r) 32,000 dimeric form and the monomeric forms of MEIG1 remain cytoplasmic. The appearance in the nuclear fraction is developmentally regulated, coinciding with progression of the first spermatogenic wave through meiotic prophase I. Utilizing immunocytochemistry we show that nuclear localization is apparent in primary spermatocytes through their maturation into elongated spermatozoa, but not in either somatic cells or germ cells from early postnatal pups. We also show that MEIG1 associates specifically with meiotic chromosomes in vivo. These results indicate that in germ cells, the M(r) 31,000 dimeric form enters the nucleus during the first meiotic prophase and binds to the meiotic chromatin. Possible nuclear functions, as well as possible modes of nuclear localization, are discussed.