MicroRNAs (miRNAs) are small RNAs that bind to the 3 UTR of mRNAs. We are using zebra fish as a model system to study the developmental roles of miRNAs and to determine the mechanisms by which miRNAs regulate target mRNAs. We generated zebra fish embryos that lack the miRNA-processing enzyme Dicer. Mutant embryos are devoid of mature miRNAs and have morphogenesis defects, but differentiate multiple cell types. Injection of miR-430 miRNAs, a miRNA family expressed at the onset of zygotic transcription, rescues the early morphogenesis defects in dicer mutants. miR-430 accelerates the decay of hundreds of maternal mRNAs and induces the deadenylation of target mRNAs. These studies suggest that miRNAs are not obligatory components of all fate specification or signaling pathways but facilitate developmental transitions and induce the deadenylation and decay of hundreds of target mRNAs.