In fission yeast the ability to undergo meiosis and sporulation is conferred by the matP+ and matM+ genes of the mating-type locus. Inactivation of ran1+, a negative regulator of meiosis, is thought to be an essential step in meiotic initiation. We have isolated a further meiotic control gene mei3+, and have shown the following: a null allele of mei3 totally inhibits meiosis; the mei3+ RNA transcript and its translational product are expressed only in matP+/matM+ diploids entering meiosis; forced expression of mei3+ in vegetative cells provokes haploid meiosis and sporulation. We suggest that the product of mei3+ gene, a protein of 21 kd, initiates meiosis by inactivating ran1+.