During mouse germ cell development, the first sign of sex differentiation occurs when XY germ cells enter G₁/G₀ arrest from 12.5 days postcoitum (dpc). Retinoblastoma 1 (RB1), a potent cell cycle regulator, was investigated in XY germ cell arrest by studying germ cell proliferation in Rb1^−/− mutant mouse embryos. Because mice homozygous for the Rb1 deletion die in utero around 14.5 dpc, we used ex vivo culture techniques to allow analysis of developing gonads to 16.5 dpc. In Rb1^−/− gonads, we observed normal somatic cell development, assessed by immunofluorescence for markers HSD3B1 and anti-Müllerian hormone. However, at 14.5 dpc, when wild-type XY germ cells had arrested, we could detect actively proliferating germ cells using the proliferation markers MKI67, pHH3, and bromodeoxyuridine incorporation. The increased proliferation was reflected with a trend of increased germ cell number and expression of germ cell markers Ddx4 and Pou5f1 in the Rb1^−/− testes. By 16.5 dpc, this phenotype was resolved such that the entire germ cell population had entered G₁/G₀ arrest, although the total germ cell number remained elevated. At each stage analyzed, we saw no increase in expression of RB1 family members Rbl1 and Rbl2 in the Rb1^−/− testes, but we saw a significant increase of cyclin-dependent kinase (CDK) inhibitor Cdkn1b and Cdkn2b expression. We conclude that Rb1 is required for correct germ cell entry into G₁/G₀ arrest in the wild-type gonad at 14.5 dpc, but in its absence, upregulation of other cell cycle suppressors, including Cdkn1b and Cdkn2b, can induce delayed germ cell arrest.