MicroRNAs (miRNAs) comprise a large family of regulatory molecules that repress protein production from targeted mRNAs. Although it is now clear that miRNAs exert pervasive effects on gene expression in animal cells, the mechanism(s) by which they function remains poorly understood. We have analyzed the subcellular distribution of miRNAs in actively growing HeLa cells and find that the vast majority are associated with actively translating mRNAs in polysomes. We also find that a specific miRNA-regulated mRNA (KRAS) is polysome associated and that its translation is impaired, apparently at the level of elongation. These observations are discussed in light of our current understanding of mechanism of miRNA function.