The authors report on the realization of an electro-optical amplitude modulator based on Β-Ba B2 O4 (BBO) waveguide operating in the ultraviolet wavelength range down to 257 nm. The ridge waveguides are produced by He+ implantation, photolithography masking, and plasma etching. Electrodes are deposited by the standard lift-off technique. A minimum half-wave voltage times modulator length Vπ ×1 of 43 V cm has been demonstrated at 257 nm in a 6.5-mm -long modulator. Because of BBO's broad transparency range (190-3500 nm), such device could be employed in the deep ultraviolet with Vπ ×1 estimated to reach 24 V cm at 200 nm.