In Caenorhabditis elegans, lin-4 and let-7 encode 22- and 21-nucleotide RNAs, respectively, that function as key regulators of developmental timing. Because the appearance of these short RNAs is regulated during development, they are also referred to as "small temporal RNAs" (stRNAs). We show that many more 21- and 22-nt expressed RNAs, termed microRNAs (miRNAs), exist in invertebrates and vertebrates, and that some of these novel RNAs, similar to let-7 stRNA, are also highly conserved. This suggests that sequence-specific post-transcriptional regulatory mechanisms mediated by small RNAs are more general than previously appreciated.