A sensitive and novel DNA electrochemical biosensor for the detection of the transgenic plants gene fragment by electrochemical impedance spectroscopy (EIS) was presented. The well-dispersed carboxylic group-functionalized single-walled carbon nanotubes (SWNTs) were dripped onto the carbon paste electrode (CPE) surface firstly, and poly-L-lysine films (pLys) were subsequently electropolymerized by cyclic voltammetry (CV) to prepare pLys/SWNTs/CPE. The morphology of pLys/SWNTs films was examined using a field emission scanning electron microscope (SEM). The pLys/SWNTs films modified electrode exhibited very good conductivity. DNA probes were easily immobilized on the poly-L-lysine films via electrostatic adsorption. The hybridization events were monitored with electrochemical impedance spectroscopy using [Fe(CN)(6)](3-/4-) as indicator. The PAT gene fragment from phosphinothricin acetyl transferase gene was detected by this DNA electrochemical sensor. The dynamic detection range of this sensor to the PAT gene fragment was from 1.0 x 10(-12) to 1.0 x 10(-7) mol/L. A detection limit of 3.1 x 10(-13) mol/L could be estimated. The PCR amplification of NOS gene from the sample of a kind of transgenic modified bean was also detected satisfactorily by EIS. (C) 2007 Elsevier Ltd. All rights reserved.