A simple method for the nanostructuring of polyaniline (PANI)-modified screen-printed electrodes (SPE) was developed. Polystyrene (PS) nanoparticles were self-assembled onto a PANI-modified SPE with the help of the negatively charged polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS) and positively charged poly (diallyldimethylammonium chloride) (PDDA). The self-assembled PS nanoparticles could then act as a template for the electropolymerization of an additional layer of PANI. It was found that the additional layer of PANI preferred to grow around the PS nanoparticles, and a highly nodular, 'cauliflower-like' nanostructure of PANI was thus formed on the electrode surface, which was characterized by scanning electron microscopy. The SPE modified with the nanostructured PANI (nano-PANI) was further used for the immobilization of horseradish peroxidase (HRP) and developed into a biosensor for hydrogen peroxide. Under optimized conditions, the proposed biosensor exhibited a fast response time of about 5 s, a linear range from 2.5 x 10(-6) to 1.9 x 10(-4) mu M with a detection limit of 0.36 mu M, and a high sensitivity of 41.0 mu A mM(-1) due to the benefits of the nanostructured PANI.