A muon-spin-rotation study of the flux-line lattice in is presented. For the field parallel to the crystallographic c-direction, the observed field distribution strongly indicates a square symmetry of the vortex lattice. We determine the value of the coherence length from the upper critical field and the Ginzburg-Landau parameter which is found to be from the field distribution. This gives a value for the penetration depth of . The temperature dependence of the penetration depth is measured.