Ocean seismic layer 3 is distinguished from layer 2 by higher velocities, lower variability, and lower gradients in velocity with depth. Based on studies of rocks recovered from ophiolites and walls of fracture zones, most models of lower ocean crust correlate seismic layer 3 with sequences of gabbroic lithologies. The average velocity (6.5 km/s) for a vertical seismic profile in a gabbro sequence at Ocean Drilling Program (ODP) Hole 735B is consistent with refraction values for layer 3. However, the Q (inverse attenuation) obtained from these data, after correction to temperature and pressure conditions in lower crust, are too low to be consistent with amplitudes of signals observed in refraction experiments. Laboratory measurements of Q on gabbros from Hole 735B and ophiolites are also one to two orders of magnitude lower than seismic refraction Q. These results indicate that the gabbro sequence at Hole 735B cannot be typical of seismic layer 3. Based on Q, upper layer 3 may be metadolerite, and lower layer 3 may include interbedded gabbros and ultramafics. Serpentine is highly attenuating and is unlikely to be a major component. ¿ American Geophysical Union 1992