We have identified a Drosophila homolog of the DNA replication initiation inhibitor Geminin (Dm geminin) and show that it has all of the properties of Xenopus and human Geminin. During Drosophila development, Dm Geminin is present in cycling cells; protein accumulates during S phase and is degraded at the metaphase to anaphase transition. Overexpression of Dm geminin in embryos inhibits DNA replication, but cells enter mitosis arresting in metaphase, as in dup (cdt1) mutants, and undergo apoptosis. Overexpression of Dm Geminin also induces ectopic neural differentiation. Dm geminin mutant embryos exhibit anaphase defects at cycle 16 and increased numbers of S phase cells later in embryogenesis. In a partially female-sterile Dm geminin mutant, excessive DNA amplification in the ovarian follicle cells is observed. Our data suggest roles for Dm Geminin in limiting DNA replication, in anaphase and in neural differentiation.