Preparation of a polypyrrole nanofiber (PPyNF) modified pencil graphite (PG) electrode and its usage in the electrochemical DNA biosensors was investigated. The electrodes (PPyNF/PG1 and PPyNF/PG2) were prepared from a solution containing 0.1 M pyrrole, 0.1 M Na2CO3 and 0.1 M LiCIO4 by using potentiostatic and potentiodynamic methods. PPvNF/PG2 electrodes which were prepared by potentiostatic procedure showed higher responses for the oxidation of ds-DNA than the PPyNF/PG1 electrodes prepared by potentiodynamic methods. Immobilization of the ds-DNA on PG and PPyNF/PG surfaces was performed at a constant potential, +0.5 V for 300 s in 0.5 M ABS (pH 4.8) containing 15 mu g mL(-1) ds-DNA and 20 mM LiCIO4. The oxidation peak potentials of the ds-DNA bases, guanine and adenine, were shifted to more cathodic values by using PPyNF/PG electrodes. The oxidation signal of the guanine base of ds-DNA was decreased in the presence of methylene blue.