Design and operation of a quartz fiber cytoanalytical balance are described. The 2-µ fiber, 3-4 mm long, is mounted in a V-shaped notch in an aluminum slide for use on an ordinary microscope. The deflection is measured with the focusing adjustment by viewing through thin glass windows, which are flush with the surfaces of the slide. Since the opening to the balance faces away from the operator, weighings can be made without closing the balance and removing time manipulator needles employed for handling the specimens. Transverse sections of single muscle cells, 0.1 µg after freeze-drying, can be weighed to within 3% under ordinary atmospheric conditions.