In this paper a new third-generation amperometric biosensor for lactose determination is described. The biosensor is based on the highly efficient direct electron transfer (DET) between cellobiose dehydrogenase (CDH) from Phanerochaete sordida (PsCDH) and single walled carbon nanotubes (SWCNT). The SWNCTs were surface modified with aryl diazonium salts of p-phenylenediamine (NH2-PD) and deposited on top of a glassy carbon (GC) electrode. The PsCDH NH2-PD/SWCNT-GC biosensor showed very efficient DET and exhibited an extraordinary high current density of 500 mu M cm(-2) in a 5 mM lactose solution at pH 3.5. The biosensor has a detection limit for lactose of 0.5 mu M, a large linear range from 1 to 150 mu M lactose and a high sensitivity (476.8 nA mu M-1 cm(-2)). It shows also a fast response time (4s), good reproducibility (RSD =1.75%) and good stability (half-life 12 days). In addition, it is easy, simple to manufacture, and cheap because a low amount of enzyme is required and highly selective, as no significant interference was observed. For these reasons, it can represent a valid alternative to HPLC measurements for lactose determination in milk and dairy products. (c) 2012 Elsevier B.V. All rights reserved.