Two magnetotelluric profiles have been recorded in the state of Washington in order to study crustal structures related to the Cascade Range volcanoes. A long magnetotelluric sounding profile was completed from near the Pacific Coast to the eastern part of Washington, covering some 400 km in length. This profile crossed the Cascade Range midway between Mount Rainer and Mount Adams. A shorter east-west profile of data from southwest of Mount St. Helens to southeast of Mount Adams has also been interpreted. The west end of the long period from near the Pacific Coast to the vicinity of Mount Rainier can be interpreted to indicate as much as 10 km of conductive Tertiary and possibly pre-Tertiary sedimentary and volcanic rocks. These rocks have resistivities of 2--10 ohm m as demonstrated on the magnetotelluric soundings and electrical well logs. The interpretations show that these conductive materials are juxtaposed with a pluton in the Goat Rocks Wilderness, southeast of Mount Rainier. This feature and other characteristics on the shorter profile to the south are interpreted to be the cause of geomagnetic variation anomalies observed in the Washington Cascades in other research. The thick conductive units on the west end of the long profile and other assumed sedimentary units detected beneath basalts of the Columbia River region have significance in regional hydrocarbon exploration programs. Displacements in the thick conductive unit occur across the Mount St. Helens seismic zone defined by C. S. Wever and the general pattern of seismicity in the Puget Lowland and south may be related to the interpreted sedimentary unit. Major differences in crustal electrical structures were observed in the Washington State volcanic Cascades compared to the High Cascades in Oregon and northern California. The main features of the magnetotelluric interpretations can be coupled to offshore seismic and gravity data to provide better constraints on the nature of paleosubduction of the Juan de Fuca plate.