This paper describes a method for manipulating and monitoring the rotational motion of single, optically trapped microparticles and living cells in a microvortex. To induce rotation, we placed the microparticle at the center of rotation of the vortex and used the recirculating fluid flow to drive rotation. We have monitored the rotation of single beads (which ranged in diameter from a few micrometers to tens of micrometers) and living cells in a microvortex. To follow the rotation of a smooth and symmetrically shaped bead, we first ablated a small region ( approximately 1 microm) on the bead. An Ar(+) laser was then tightly focused ( approximately 0.5-microm spot size) onto the bead, and rotation was tracked by recording changes in the level of backscattered laser light as the ablated region repeatedly transited the laser focus. Using this method, we have followed bead rotation that varied in frequency from 0.15 to 100 Hz and have studied the effect of bead diameter on the rate of rotation at a given fluid flow rate. To monitor the rotation of single living cells, we selectively stained portions of B-lymphocytes with the fluorescent dye DiOC(6). We observed rotation by following changes in the fluorescence signal as the dye-stained region transited the laser focal volume. This technique provides a simple and sensitive method for controlling and monitoring the rotational motion of microparticles in a microfluidic environment.