A quadrant photodetector has been added onto a custom differential interference contrast microscope to measure two-axis displacement of silica beads with a noise ⩽0.1 nm/&sqrt;Hz below 150 Hz. A diode-pumped solid-state laser is incorporated to function not only as a light source for displacement detection but also as an optical trap for motility studies of protein motors. This system is designed for studies in which the motility needs to be simultaneously monitored along both axes at nm accuracy. ©1997 American Institute of Physics.