The conserved lin-4 microRNA (miRNA) regulates the proper timing of stem cell fate decisions in C. elegans by regulating stemness genes such as lin-14 and lin-28.1-3 While lin-4 is upregulated toward the end of the first larval stage and functions as an essential developmental timing “switch”, little is known about how lin-4 expression is regulated.4 Here we show that in C. elegans hypodermal seam cells, transcription of lin-4 is positively regulated by lin-4 itself. In these cells, lin-4 activates its own transcription through a conserved lin-4-complementary element (LCE) in its promoter. We further show that lin-4 is required to recruit RNA polymerase II to its own promoter, and that lin-4 overexpression is sufficient for autoactivation. Finally, we show that a protein complex specifically binds the LCE in vitro, and that mutations that abolish this binding also reduce the in vivo expression of a plin-4:GFP reporter. Thus, we describe the first in vivo evidence of RNA activation (RNAa) by an endogenous miRNA, and provide new insights into an elegant autoregulatory mechanism that ensures the proper timing of stem cell fate decisions in development.