Based on the synchronization between a periodic vibration and flashing light, we designed a device to determine the flagellar beating frequency of human sperm. The head of a spermatozoon was either held by the tip of a micropipette operated with a micromanipulator or adhered by itself on the surface of a glass slide when the sperm swam out of the micropipette into the fresh Ham's F-10 medium. The beat frequency of the flagellum was measured by synchronization of the frequency-adjustable flash light built on an inverted microscope. The light frequency synchronizer included a controller, a pulse generator, a signal counter, and a flash illuminator. During each measurement, the spermatozoon was transferred to the center of the observing field and the frequency generator created a series of signals which developed flashing signals onto the sample plane. When the vibration of the flagellum was observed as a constant two-step-like movement after frequency adjustment, the beating frequency was read from the signal counter and the count was twice that of the beat frequency of the sperm tail. As the flash signal was decreased to half of this frequency, an apparently immobilized sperm flagellum was observed and the exact beating frequency could then be determined. This device was then used to measure the effect of pentoxifylline on sperm motility. The results showed that the increase of tail beat frequency as measured by this device is well correlated with the increase of beat cross frequency as detected by a computer-assisted semen analyzer. These findings suggest that this flash light synchronization device is a reliable and useful system for the assessment of sperm motility.