Using a 2.3 mm thick KBe(2)BO(3)F(2) crystal optically contacted with CaF(2) in a prism-coupled device, fourth-harmonic generation of a femtosecond (150 fs) Ti:sapphire laser system at wavelength 200 nm with a high conversion efficiency of 26.1% has been recently obtained. The maximum output at 200 nm is 10.7 mW.