Shallow subsurface electrical conductivity was mapped at Bigslough National Wildlife Refuge (NWR) in northeast Montana using the DUALEM421 electromagnetic sensor (Dualem, Inc., ON, Canada) in the winter of 2017. Data were acquired by towing the DUALEM421 sensor on a sled behind an all-terrain vehicle or snow machine, with the sensor at a nominal height of 0.3 meters (m) above ground surface. Approximately 36 line-kilometers (km) of data were acquired over an area of approximately 2 square-kilometers. Data were manually edited to remove sensor dropouts, lag corrected for apparent offsets between recorded GPS location and data locations for each coil pair, and averaged to a sounding distance of 1m along the survey path; data were not decoupled from infrastructure noise sources (powerlines, pipelines, fences, etc.) or corrected for sensor pitch or roll. This data release contains raw and processed electromagnetic data. Digital data are described by the data dictionaries. Additional details regarding the processing steps are described in the metadata.