  We probed the bending of actin subject to external forcing and viscous drag.
Single actin filaments were moved perpendicular to their long axis in an
oscillatory way by means of an optically tweezed latex bead attached to one end
of the filaments. Shapes of these polymers were observed by epifluorescence
microscopy. They were found to be in agreement with predictions of semiflexible
polymer theory and slender-body hydrodynamics. A persistence length of $7.4 \pm
0.2 \mu$m could be extracted.
