Lymphoid-specific helicase (HELLS; also known as LSH) is a member of the SNF2 family of chromatin remodeling proteins. Because Hells-null mice die at birth, a phenotype in male meiosis cannot be studied in these animals. Allografting of testis tissue from Hells^−/− to wild-type mice was employed to study postnatal germ cell differentiation. Testes harvested at Day 18.5 of gestation from Hells^−/−, Hells ^/−, and Hells ^/ mice were grafted ectopically to immunodeficient mice. Bromodeoxyuridine incorporation at 1 wk postgrafting revealed fewer dividing germ cells in grafts from Hells^−/− than from Hells ^/ mice. Whereas spermatogenesis proceeded through meiosis with round spermatids in grafts from Hells heterozygote and wild-type donor testes, spermatogenesis arrested at stage IV, and midpachytene spermatocytes were the most advanced germ cell type in grafts from Hells^−/− mice at 4, 6, and 8 wk after grafting. Analysis of meiotic configurations at 22 days posttransplantation revealed an increase in Hells^−/− spermatocytes with abnormal chromosome synapsis. These results indicate that in the absence of HELLS, proliferation of spermatogonia is reduced and germ cell differentiation arrested at the midpachytene stage, implicating an essential role for HELLS during male meiosis. This study highlights the utility of testis tissue grafting to study spermatogenesis in animal models that cannot reach sexual maturity.