We have determined the 3D structure of a 34-nt RNA construct, herein named LCS1co, which mimics the interaction of let-7 microRNA (miRNA) to one of its complementary binding sites, LCS1, in the 3'-untranslated region of lin-41 mRNA by solution-state NMR spectroscopy. let-7 miRNAs control the timing of development of the nematode Caenorhabditis elegans and are highly conserved in mammals. The sequence and structure of the two conserved let-7 complementary sites, LCS1 and LCS2, in the 3'-untranslated region of lin-41 mRNA are important for a proper downregulation of lin-41. The high-resolution NMR structure reveals details of the binding of let-7 miRNA to lin-41 mRNA which involves formation of a complex with non-canonical structural elements within the seed region. LCS1co exhibits a stem-loop structure with two stems, an asymmetric internal loop and an adenine bulge. Comparison with the NMR solution-state structure of the let-7:lin-41 complex involving the LCS2-binding site shows that conformational freedom of the asymmetric internal loop of LCS1co correlates with a smaller bend between the upper and lower stems in comparison to the well-defined asymmetric loop of LCS2co.