This paper describes a novel technique for monitoring the outside dimensions of clad or unclad optical fibers. An oscillating mirror is used to deflect a laser beam with constant velocity across a fiber in order to measure the time interval during which the fiber intercepts the beam and casts a shadow on a photodetector. For 110 μ fibers, individual measurements made every 1/60 sec are repeatable within a spread of ± 1% from the 330 μsec mean time interval (rms deviation ≈ ± 0.6%). Results are displayed in digital form, but can be converted to an analog signal for use in a servo loop to control the pulling rate of fiber drawing machines. An additional feature of the technique is that fibers with noncircular cross sections can be monitored by making sequential diameter measurements along orthogonal axes of the fiber.