Poly(C)-binding proteins (CPs) are important regulators of mRNA stability and translational regulation. They recognize C-richRNA through their triple KH (hn RNP K homology) domain structures and are thought to carry out their function though direct protection of mRNA sites as well as through interactions with other RNA-binding proteins. We report the crystallographically derived structure of the third domain of alpha CP1 to 2.1 angstrom resolution. alpha CP1-KH3 assumes a classical type I KH domain fold with a triple-stranded beta-sheet held against a three-helix cluster in a beta alpha alpha beta beta alpha configuration. Its binding affinity to an RNA sequence from the 3'-untranslated region (3'-UTR) of androgen receptor mRNA was determined using surface plasmon resonance, giving a K-d of 4.37 mu M, which is indicative of intermediate binding. A model of alpha CP1- KH3 with poly(C)-RNA was generated by homology to a recently reported RNA-bound KH domain structure and suggests the molecular basis for oligonucleotide binding and poly(C)-RNA specificity.