Two-dimensional (2-D) electrical resistivity tomography data were acquired across and in proximity to active sinkholes at two highway construction sites in Missouri. Site 1 is located in Greene County in southwestern Missouri; site 2 is located in Jefferson County in east-central Missouri. Eleven 2-D electrical resistivity tomography profiles were acquired along a suite of parallel traverses (spaced at 7.5 m intervals) at site 1 as part of the geotechnical investigation of a proposed interchange. The primary objectives were to identify and map air-filled karstic cavities (if present) and the top of bedrock. The resistivity data from site 1 were processed as both conventional 2-D data and as three-dimensional (3-D) data using two different inversion schemes. The top of bedrock was imaged, and five prominent north-trending, clay-filled, solution-widened fractures were identified. Interpreted air-filled voids were not imaged. At site 2, a pattern of 17 parallel electrical resistivity tomography profiles was acquired across and in proximity to a preexisting sinkhole (approximately 15 m × 10 m × 5 m L/W/D). During construction of the highway, the soil was emptied from the sinkhole overnight following a heavy rainstorm. Electrical resistivity tomography data were acquired to determine if the sinkhole was underlain by a large air-filled cavity. Interpretation of the 2-D and 3-D resistivity images from site 2 showed that the sinkhole was not underlain by a substantive cavity. Rather, the soil in the sinkhole appears to have “flowed” into the subsurface through solution-widened fractures that do not pose a significant risk to the overlying roadway.