We report on the observation of single W boson production in a data sample collected by the L3 detector at LEP2. The signal consists of large missing energy final states with a single energetic lepton or two hadronic jets. The cross-section is measured to be 0.61−0.33+0.43 ± 0.05 pb at the centre of mass energy √s = 172 GeV, consistent with the Standard Model expectation. From this measurement the following limits on the anomalo γWW gauge couplingsare derived at 95% CL: −3.6 < Δκγ < 1.5 and −3.6 < λγ < 3.6.