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This paper describes an optical time-domain reflectometer that incorporates a gated photomultiplier receiver. The instrument can detect extremely weak reflections from fiber breaks (more than 65 dB below the 4-percent reflection of a perfect break) with 0.5-m distance resolution. In addition, backward Rayleigh scattering, which occurs roughly uniformly along a fiber, can be used to estimate the attenuation vs position within a fiber. Therefore, regions of high attenuation can be located nondestructively from one end of the fiber.