We here report on a surface plasmon resonance (SPR) sensor carrying small organic ligands for the detection of single-nucleotide polymorphisms (SNPs). Two kinds of ligands are prepared, both of which have a hydrogen-bond forming site suitable for nucleobase recognition, and have an active amino group for the immobilization to the sensor chip. While the sensor immobilized flavin does not show any useful responses, the sensor based on 3,5-diaminopyrazine shows a highly selective response to thymine over other nucleobases opposite an abasic site in DNA duplexes (5'-GTT GGA GCT GXG GGC GTA GGC-3'/3'-CAA CCT CGA CNC CCG CAT CCG-5', X = AP site, N = target; G, C, A, T). In PBS buffer (pH 6.4, 0.25 M NaCl, at 5 degrees C), the sensor can detect 10 nM of the sample solution, and the SPR signal for thymine is linear in the concentration range from 10 nM to 100 nM. These sensing functions of the present sensor are discussed for the development of SNPs detection chemistry based on DNA-binding small molecules.