A Langmuir probe measurement system suitable for characterization of extended, low‐pressure dc or microwave plasmas has been developed around a VAXStation 3200 graphics workstation computer. Both planar and cylindrical probes have been constructed, but the latter has been deemed more satisfactory, and its design developed more fully. A novel interface circuit based on fast high voltage, transformer‐coupled, isolation amplifiers has been constructed to permit probing of regions where the plasma potential is hundreds of volts away from ground, which are typically found in dc glow discharges. This circuit also incorporates a noise suppression feature, using feedback from a second identical probe (compensation probe), which has proven very useful. A great deal of high‐resolution data can be collected quickly using the fast direct memory access (DMA) hardware. Many analysis capabilities and graphical display possibilities are implemented in the FORTRAN control program, but special emphasis has been placed on extracting plasma potentials, electron densities, and electron energy distribution functions (EEDFs) from the first and second derivatives of the probeI‐Vcurve. These are obtained by numerical differentiation techniques that fully correct for ohmic voltage drops across the current sampling resistor. The capabilities of the system will be illustrated with several examples of probe data and EEDFs obtained in a large dc glow discharge system.