We present a method for measuring molecular-beam epitaxy growth rates in near real-time on rotating substrates. This is done by digitizing a video image of the reflection high-energy electron diffraction screen, automatically tracking and measuring the specular spot width, and using numerical techniques to filter the resulting signal. The digitization and image and signal processing take approximately 0.4 a to accomplish, so this technique offers the molecular-beam epitaxy grower the ability to actively adjust growth times in order to deposit a desired layer thickness : The measurement has a demonstrated precision of approximately 2%, which is sufficient to allow active control of epilayer thickness by counting monolayers as they are deposited. When postgrowth techniques, such as frequency domain analysis, are also used, the reflection high-energy electron diffraction measurement of layer thickness on rotating substrates improves to a precision of better than 1%. Since all of the components in the system described are commercially available, duplication is straightforward.