In this article, we show that a subset of conventional mammalian microRNAs is derived from LINE-2 transposable elements and other genome repeats. These repeat-derived microRNAs arise from conventional precursor hairpins and are distinct from the rasiRNAs, which appear to be processed from long double-stranded RNA precursors. The insertion of transposable elements into new genomic sites appears to be one of the driving-forces that create new microRNAs during mammalian evolution. Two of the LINE-2-derived microRNAs exhibit perfect complementarity to a large family of mRNA and EST transcripts that contain portions of MIR and other LINE-2 elements in their 3'-untranslated regions.