Wide-wavelength-band operation of InP-based Mach-Zehnder modulators is investigated. From the measured dependences of the half-wavelength voltage on operating wavelength and applied-bias voltage, it is shown that wide-wavelength operation with a constant driving voltage can be realized by adjusting the bias voltage for each operating wavelength. Ten-gigabits/second clear eye openings are demonstrated over a 30-nm wavelength range with a constant driving voltage of 2 V/sub pp/.