We demonstrate for the first time cascaded wavelength conversion by four-wave mixing in a semiconductor optical amplifier. Bit-error-rate performance of <10/sup -9/ at 10 Gb/s is achieved for two conversions of up to 9 nm down and up in wavelength. For two wavelength conversions of 5 nm down and up, a power penalty of 1.3 dB is measured. A system of two wavelength converters spanning 40 km of single-mode fiber is also demonstrated.