Erbium-doped fiber amplifiers (EDFA) for use in bidirectional optical transmission systems requiring no or only a single repeater amplifier are investigated. Signal directions are discriminated by using two wavelengths: 1.535μm and 1.55μm. The optical gain difference between the two wavelengths is compensated by a subsidiary erbium-doped fiber installed in the 1.55μm channel. Each EDFA for the terminal equipment is designed to serve both as a postamplifier and as a preamplifier. The repeater amplifier, consisting of a unidirectional EDFA and a bridge circuit, combines and then separates the opposite directional signals. The power buget margins for a bit error rate of 10^-9 at 2.488Gbit/s are 49dB for non-repeater transmission and more than 42dB at both sides of the repeater for one-repeater transmission.