Variable-wavelength photodetectors are fabricated by a selective growth of Zn_xCd_1−xSe alloy nanowires on patterned Au catalysts thus forming nanowire air-bridges between two Pt pillar electrodes. From the composition-dependent linear changes of bandgap energies and lattice parameters, Zn_xCd_1−xSe nanowires are found to be perfectly alloyed in the entire range of Zn composition without any phase separation and have a structural transition from zinc blende to wurtzite at 0.31 < x < 0.72. The spectral responses of Zn_xCd_1−xSe detectors show that the cut-off wavelength can be continuously tuned within the visible spectrum of light extending from red to blue. In on-off switching operations, all of the detectors can be reversibly switched between the low and the high conductivity state but the CdSe detector has slower response and recovery behaviors than ZnSe and Zn_0.72Cd_0.28Se ones, which is explained by adsorption and photodesorption of O₂ on the surface of CdSe nanowires.