Basonuclin is a zinc-finger protein found in abundance in oocytes. It qualifies as a maternal-effect gene because the source of pre-implantation embryonic basonuclin is maternal. Using a transgenic-RNAi approach, we knocked down basonuclin specifically in mouse oocytes, which led to female sub-fertility. Basonuclin deficiency in oocytes perturbed both RNA polymerase I- and II-mediated transcription, and oocyte morphology was affected (as evidenced by cytoplasmic and cell surface abnormalities). Some of the affected oocytes, however, could still mature to and arrest at metaphase II, and be ovulated. Nevertheless, fertilized basonuclin-deficient eggs failed to develop beyond the two-cell stage, and this pre-implantation failure accounted for the sub-fertility phenotype. These results suggest that basonuclin is a new member of the mammalian maternal-effect genes and, interestingly, differs from the previously reported mammalian maternal-effect genes in that it also apparently perturbs oogenesis.