Abstract. An embedding of the group Diff(S 1 ) of orientation preserving diffeomorphims of the unit circle S 1 into an infinite-dimensional symplectic group, Sp(∞), is studied. The authors prove that this embedding is not surjective. A Brownian motion is constructed on Sp(∞). This study is motivated by recent work of H. Airault, S. Fang and P. Malliavin.