miRNAs are a class of versatile small RNAs that control gene expression post-transcriptionally, governing many facets of plant cell functions. They interact with their target mRNA at a site of sequence complementarity and modulate their expression levels. Here, we provide evidence, based on transient assays and stable transgenic lines, that the 3′ UTR of the Arabidopsis SBP box geneSPL3contains a functional miRNA-responsive element (MRE) that is complementary to miR156 and miRNA157. Seedlings of transgenic lines constitutively over-expressing anSPL3transgene either carrying an unaltered or a disrupted MRE accumulate considerable levels ofSPL3transcripts. However, while the unaltered MRE UTR does not allow the expression of detectable levels of SPL3 protein, the altered MRE does. Translational inhibition thus provides an important mechanism for miRNA-mediated post-transcriptional repression ofSPL3. As a consequence of precocious translation of the constitutively expressedSPL3transgene, due to the absence of a functional MRE, plants exhibit very early flowering in addition to frequent morphological changes.