A transient tunable Bragg reflection grating has been established in an erbium-doped fiber amplifier by the effect of a standing wave produced by counterpropagating cw light. Less than 1 mW of cw light was required to establish a grating with a 75% reflection coefficient at 1536 nm and an optical bandwidth (FWHM) of 16 MHz. Possible applications include dispersion compensation and dense wavelength-division multiplexing.