Spin density matrix elements were extracted from COMPASS data for exclusive \rho^{0} ρ 0 meson muoproduction on a liquid hydrogen target. The measurement cover the kinematic range of 1.0 (GeV/ c c ) ^2 2 < Q^{2} < < Q 2 < 10.0 (GeV/ c c ) ^2 2 , 5.0 GeV/ c^2 c 2 < W < < W < 17.0 GeV/ c^2 c 2 and 0.01 (GeV/ c c ) ^2 2 <p^{2}_{T}< < p T 2 < 0.5 (GeV/ c c ) ^2 2 . Here, W W denotes the mass of the final hadronic system, Q^{2} Q 2 the virtuality of the exchanged photon and p_{T} p T the transverse momentum of the \rho^{0} ρ 0 meson with respect to the virtual-photon direction. We observe a violation of s-channel helicity conservation for the transition \gamma^{*}_{T} \to \rho^{0}_{L} γ T * → ρ L 0 . Additionally, we find a dominant contribution of the natural-parity-exchange transitions.