Seismic refraction velocities provide an important contraint on the composition of the lower continental crust. However, valid inferences on the composition may require allowance for the effect of porosity. Many refraction velocities obtained for the lower crust are characteristic of laboratory values for intermediate composition rocks, in disagreement with other estimates of a primary mafic lower crust composition. The discrepancy is particularly large in Phanerozoic areas where most velocities are less than 7.0 km s-1, and where non-seismic estimates such as from xenoliths are almost exclusively mafic. A mafic composition can be reconciled with the refraction data if the in situ velocities are reduced by porosity of a few percent. Support for the model is given by magnetotelluric measurements which have shown the lower crust to be commonly very electrically conductive. Lower crustal porosities of up to several percent are inferred, particularly in areas where the resistivity is less than 100 ohm-m. ¿ American Geophysical Union 1989