The elucidation of microRNA function and evolution depends on the identification and characterization of miRNA repertoire of strategic organisms, as the fast evolving cichlid fishes. Using RNA-seq and comparative genomics we carried out an in-depth report of miRNAs in Nile tilapia (Oreochromis niloticus). Our results enlarge vertebrate miRNAs collection and reveal a notable differential expression of miRNAs arms and isoforms influenced by sex and developmental life stage, providing a better picture of the evolutionary and spatiotemporal dynamics of miRNAs. Overall design: Discovery and evolutionary analysis of microRNAs