The mouse meiotic mutant Mei1 was isolated in a screen for infertile mice descended from chemically mutagenized embryonic stem cells. Homozygotes of both sexes are sterile due to meiotic arrest caused by defects in chromosome synapsis. Notably, RAD51 protein does not load onto Mei1 mutant meiotic chromosomes, suggesting that there is a defect in either recombinational repair or the production of double-strand breaks (DSBs) that require such repair. Here, we show that treatment of mutant males with cisplatin restores RAD51 loading, suggesting that mutant spermatocytes have intact recombinational repair mechanisms. Levels of histone H2AX phosphorylation (gammaH2AX) at leptonema are significantly reduced compared with wild-type controls but comparable to that seen in animals deficient for SPO11, the molecule required for catalyzing DSB formation during meiosis. These observations provide evidence that genetically programmed DSB induction is defective in Mei1 leptotene spermatocytes. We also report the positional cloning of Mei1, which encodes a product without significant homology to any known protein. Expressed almost exclusively in gonads, Mei1 has no apparent homologs in yeast, worms, or flies. However, Mei1 orthologs are present in the genomes of mammals, chickens, and zebrafish. Thus, Mei1 is required for vertebrate meiosis. To our knowledge, Mei1 is the first meiosis-specific mutation identified by forward genetic approaches in mammals.