We report the first detection of cesium ( = 55) in the atmosphere of a white dwarf. Around a dozen absorption lines of Cs , Cs , and Cs have been identified in the Far Ultraviolet Spectroscopic Explorer spectrum of the He-rich white dwarf HD 149499B ( eff = 49,500 K, log = 7.97). The lines have equivalent widths ranging from 2.3 to 26.9 mÅ. We performed a spectral synthesis analysis to determine the cesium content in the atmosphere. Non-LTE atmosphere models were computed by considering cesium explicitly in the calculations. For this purpose we calculated oscillator strengths for the bound-bound transitions of Cs -Cs with both (multiconfiguration Breit-Pauli) and 2 (multiconfiguration Dirac-Fock) atomic structure codes as neither measured nor theoretical values are reported in the literature. We determined a cesium abundance of log (Cs)/ (He) = −5.45 ± 0.35, which can also be expressed in terms of the mass fraction log Cs = −3.95 ± 0.35.