A novel DNA electrochemical biosensor is described for the detection of neomycin phosphotransferase gene (NPT-II), a selection marker for transgenic plants. A thiol-modified capture probe immobilized onto the surface of a gold electrode and a biotinylated signaling probe were designed to be complementary to target regions of NPT-II flanked by PCR primers to eliminate false-positive signals from non-specific PCR products. The electrochemical assay of hybrids on the electrode surface was evaluated by means of both cyclic voltammetry (CV) and square wave voltammetry (SWV) after the coupling of biotinylated catalase with streptavidin-modified hybrids based on dendritic signal magnification and subsequent formation of polymerized aniline (PAn). It has been revealed that the sensor showed a linear increase within the target concentration (1.0-100 x 10(-6) mmol L-1). The limit of detection was about 0.2 x 10(-6) mmol L-1 and the specificity was enhanced significantly.