A commercially available atomic force microscope (AFM) equipped with a hand made simple self-oscillation circuit was used in imaging biomolecular samples in liquid environments, i.e. under physiological conditions. Assembled tau proteins, which are the major component of the neurofibrillary deposits in Alzheimer's disease, was taken as a trial sample. In order to image its native structure, the protein was physically absorbed on a cleaved mica surface without fixation. Using the frequency feedback imaging with a self-oscillation technique, the structure of protein fibers was clearly imaged even in a wide scanning range (3.75 mum) with a contact force less than 100 pN. Furthermore, no damage of the proteins was observed in successive imagings. This indicates that the deformation of proteins was negligible in our method. In contrast, the proteins were destroyed when the vertical applied force of above 300 pN was applied using the amplitude feedback imaging with the self-oscillation technique. (C) 2002 Elsevier Science B.V. All rights reserved.