The effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the Xigaze ophiolite, having harzburgitic composition and a degree of serpentinization ranging from 3% to 70%. We found: i) P‐ and S‐ wave velocities are linearly correlated with serpentine content; ii) anisotropy of P‐ and S‐waves decreases with increasing serpentinization (while in fresh peridotites anisotropy is of the order of 6%, for 70% of serpentinization it drops to 1% for P‐waves, and to 2%, for S‐waves). Comparison of published laboratory velocity data in oceanic lithologies, like gabbros and dolerites, with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40% by seismic waves velocities only. This comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites (up to 40% of serpentinization) in the oceanic crust formed at slow‐spreading ridges in accordance with geologica