The U.S. Geological Survey (USGS) Geology, Geophysics and Geochemistry Science Center (GGGSC) collaborated with the USGS Data at Risk (DaR) team to preserve and release a subset of magnetotelluric data from the San Andreas Fault in Parkfield, California. The San Andreas Fault data were collected by the Branch of Geophysics, a precursor to the now GGGSC, between 1989 and 1994. The magnetotelluric data selected for this preservation project were collected in 1990 using USGS portable truck mounted systems that measure the distribution of electrical conductivity beneath the surface of the earth. Truck mounted systems of this era output data to 3.5 inch discs, from which data were recovered and transformed to binary or ASCII formats using proprietary software. This USGS data release includes the original binary and ASCII data files and derivative EDI files (Wight, 1988), a common modern format for magnetotelluric modelling, created using Python-based software developed by the DaR team. Each child item on this landing page represents a different stage of the dataset's life cycle. Data, documentation and supplemental resources are included for the original binary format, the legacy ASCII format, and the modern EDI format that the DaR software produced (Enns et al, 2018).