Here we identify a Dicer-independent miRNA biogenesis pathway that employs the slicer catalytic activity of Argonaute2 (Ago2). To uncover Dicer-independent miRNAs, we sequenced small RNAs in wild type, maternal-zygotic dicer (MZdicer) and MZago2 mutants, using zebrafish as a model system. We find that, in contrast to other miRNAs, miR-451 levels were increased in MZdicer but drastically reduced in the MZago2 mutants. We show that pre-miR-451 processing requires Ago2 catalytic activity in vivo. MZago2 mutant embryos display delayed erythrocyte maturation that can be rescued by wild type Ago2 or miR-451 duplex but not catalytically dead Ago2. We propose that Ago2-mediated cleavage of a subset of pre-miRNAs, followed by uridylation and trimming, generates functional miRNAs in a Dicer-independent manner. Overall design: Examination of small RNAs (18 to 35 nucleotides) in 3 different zebrafish genotypes (wild type, MZago2, MZdicer) at 48 hours post-fertilization.