The stall forces of processive molecular motors have been widely studied previously in vitro . Even so, in vivo experiments are required for determining the actual performance of each molecular motor in its natural environment. We report the direct measurement of light momentum changes in single beam optical tweezers as a suitable technique for measuring forces inside living cells, where few alternatives exist. The simplicity of this method, which does not require force calibration for each trapped object, makes it convenient for measuring the forces involved in fast dynamic biological processes such us intracellular traffic. Here we present some measurements of the stall force of processive molecular motors inside living Allium cepa cells.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.