The first observation of the elementary process μ-p → νμγ is reported. The muons were stopped in a purified liquid hydrogen target and captured mainly on the ortho on the ortho pμp molecule. The photons were converted to e+e- pairs which were analyzed in a medium-resolution high-acceptance magnetic spectrometer. This reaction is particularly sensitive to the pseudoscalar coupling constant, gp, which was found to be: gp(q2 = -0.88mμ2) = (10.0 ± 0.9 ± 0.3) • ga(0). This value of gp, determined from the photon energy spectrum using a perturbation theory calculation based on tree-level Feynman diagrams, is ∼ 1.5 times the theoretical value, (6.70 ± 0.18) • ga (0). © J.C. Baltzer AG, Science Publishers.