The Puf family of RNA-binding proteins directs cell fates by regulating gene expression at the level of translation and RNA stability. Here, we report that the Caenorhabditis elegans pumilio homolog, puf-9, controls the differentiation of epidermal stem cells at the larval-to-adult transition. Genetic analysis reveals that loss-of-function mutations in puf-9 enhance the lethality and heterochronic phenotypes caused by mutations in the let-7 microRNA (miRNA), while suppressing the heterochronic phenotypes of lin-41, a let-7 target and homolog of Drosophila Brat. puf-9 interacts with another known temporal regulator hbl-1, the Caenorhabditis elegans ortholog of hunchback. We present evidence demonstrating that puf-9 is required for the 3'UTR-mediated regulation of hbl-1, in both the hypodermis and the ventral nerve cord. Finally, we show that this regulation is dependent on a region of the hbl-1 3'UTR that contains putative Puf family binding sites as well as binding sites for the let-7 miRNA family, suggesting that puf-9 and let-7 may mediate hypodermal seam cell differentiation by regulating common targets.