Gene expression in early animal embryogenesis is in large part controlled post-transcriptionally. Maternally-contributed microRNAs may therefore play important roles in early development. We have elucidated a major biological role of the nematode mir-35 family of maternally-contributed, essential microRNAs. We show that this microRNA family regulates the sex determination pathway at multiple levels, acting both upstream and downstream of her-1 to prevent aberrantly activated male developmental programs in hermaphrodite embryos. The predicted target genes that act downstream of the mir-35 family in this process, sup-26 and nhl-2, both encode RNA binding proteins, thus delineating a previously unknown post-transcriptional regulatory subnetwork within the well-studied sex determination pathway of C. elegans. Genome-wide profiling of SUP-26 binding targets reveals 775 mRNAs, most of which have no known role in sex determination, suggesting that the mir-35 family may modulate numerous other pathways via regulation of sup-26. Since sex determination in C. elegans requires zygotic gene expression to read the sex chromosome karyotype, early embryos must remain gender-naïve; our findings show that the mir-35 family microRNAs act in the early embryo to function as a developmental timer that preserves naïveté and prevents premature deleterious developmental decisions.