The electrical resistivity of a large number of ocean crust basalt samples obtained by the Deep Sea Drilling Project has been studied, including the effect of temperature and pressure, the correlation with porosity, and the changes with degree of dehydration. The data are compared to those of samples from several oceanic islands and a few gabbros that might be representative of the lower oceanic crust. The geometric mean resistivity of 153 ocean crust basalts, seawater saturated, at 22¿C and atmospheric pressure is 190 ohm m. There is no significant change with the low-temperature weathering of halmyrolysis characteristic of the upper part of old oceanic crust. Higher-temperature hydrothermal alteration such as that experienced by subaqueous Bermuda lavas appears to reduce the porosity and thus increase the resistivity. In contrast, subaerial basalts such as those from the Azores have high porosity and thus low resistivity, but Azores subaqueous samples of similar composition have the same resistivity as deep-sea basalts. The gabbros have low porosity and resistivity an order of magnitude higher than the basalts. The basalt resistivities correlate directly with porosity according to Archie's law, but with a very high slope, which suggests that pore fluid conduction is dominant but that it is affected by the presence of clay minerals. The resistivity of all saturated samples increases with increasing pressure, as is expected for pore fluid conduction. The resistivity of saturated samples decreases markedly with increasing temperature. For some of the most resistive samples, temperature dependence parallels that of seawater, but for most samples, resistivity decreases more rapidly with increasing temperature, again suggesting pore fluid conduction modified by the presence of clay minerals. Samples dried at low temperature and those dried at high temperature so that the clays are dehydrated both exhibit thermally activated conduction. In the completely dehydrated samples, conduction may be through the metallic oxide grains. The low basalt resistivities under simulated crustal conditions are in agreement with electromagnetic induction data from oceanic islands and the deep-sea floor.