The Raft River geothermal system in southeastern Idaho is a convective hot water system, presently being developed to demonstrate the production of electricity from low-temperature ( approximately e150 degrees C) water. Interpretation of seismic refraction recordings in the area yielded compressional velocities from near the surface to the crystalline basement at a maximum depth of approximately 1600 m. The results show a complex sequence of sediments and volcanic flows overlying basement. Velocities in the sedimentary section vary laterally. Correlation with well data suggests that zones of higher velocities may correspond to zones where sediments are hydrothermally altered. Flowing hot wells occur near the boundary between inferred shallow altered and unaltered rocks. The basement surface does not appear to be displaced by large faults, although there is ample surface evidence of faulting. The deep circulation of hot water necessary for a convective system may be through many small faults and fractures. Fracturing is suggested on the basis of lateral velocity variations within the basement complex.