More than 1 microW of tunable continuous-wave radiation at 369 nm has been generated by sum-frequency mixing of the radiation from a 1310-nm diode laser with the radiation from a 515-nm argon-ion laser in a beta-BaB(2)O(4) crystal. The phase-mismatching tolerance for nearly optimum focusing of input beams was found to be several times as large as that for no focusing or the calculated value. This effect was caused by relatively large double refraction of beta-BaB(2)O(4).