There is much interest in how microRNAs (miRNAs), which target mRNAs to regulate gene expression, are themselves inhibited. Now, two groups have found that a naturally expressed circular RNA (circRNA) binds to miRNAs to suppress their function. Memczak et al. and Hansen et al. show that a mammalian circRNA, termed CDR1 antisense (CDR1as) or circular RNA sponge for miR-7 (ciRS-7), contains >60 conserved miR-7 seed matches, suggesting that it can bind densely to this target miRNA. miR-7 and the circRNA are co-expressed specifically in mouse neuronal tissues, which indicates that they interact endogenously. Moreover, this circRNA associates with the miRNA effector protein Argonaute, and this is miR-7-dependent. Injection of the human variant of this circRNA into zebrafish or knockdown of endogenous miR-7 result in impaired midbrain development. This, together with the finding that this circRNA affects miR-7 target gene activity, further supports a suppressive role for circRNAs. Bioinformatic data predicting the presence of thousands of circRNAs in the genome suggest that they may function as important post-transcriptional regulators.