Results of calculations on the electrical resistivities of crustal fluids as a function of temperature, pressure and fluid composition indicate that crustal fluids can range in resistivity from approximately 1 &OHgr; m. The dominant contribution to decreasing the resistivities of crustal fluids comes from increased concentrations of chloride and bicarbonate salts and increased temperature and density of the fluids. Calculations indicate that the sharpest decrease in fluid resistivity occurs at shallow depths 10 km and temperatures >300 ¿C can not be readily explained as a product of variations in resistivities of crustal fluids. These results indicate that a transition in mineralogy or pore shape geometry is more likely responsible for the observed decrease in electrical resistivity of the middle crust than a major change in fluid resistivity. ¿ American Geophysical Union 1993