Mammalian oogenesis requires oocyte-specific transcriptional regulators. The full complement of oocyte-specific transcription factors is unknown. Here, we describe the finding that Sohlh1 , a spermatogenesis and oogenesis basic helix–loop–helix transcription factor in females, is preferentially expressed in oocytes and required for oogenesis. Sohlh1 disruption perturbs follicular formation in part by causing down-regulation of two genes that are known to disrupt folliculogenesis: newborn ovary homeobox gene ( Nobox ) and factor in the germ-line alpha ( Figla ). In addition, we show that Lhx8 is downstream of Sohlh1 and critical in fertility. Thus, Sohlh1 and Lhx8 are two germ cell-specific, critical regulators of oogenesis.