Electron microscopes using accelerating potentials of from 30 to 100 kilovolts cannot be employed satisfactorily for the examination of specimens whose thickness is greater than one‐half micron (in the case of organic material). In order to investigate the possibility of applying the high resolving power of the electron microscope to problems involving somewhat thicker specimens, an instrument has been constructed which uses electrons accelerated by voltages up to 300 kilovolts. A commercial electron microscope has been adapted for the purpose by the modification of the electron gun and projection lens, and by the addition of a new high voltage generator. The essential features of the new design and some of the problems encountered in the testing of the equipment are described. Comparison micrographs of test objects taken with 50‐ to 250‐kilovolt electrons demonstrate the increase in penetrating power obtained at the higher potentials. An electron micrograph of a biological section, one to two microns thick, taken with 200‐kilovolt electrons shows a promising amount of the internal structure.