An ultraviolet microbeam apparatus utilizing the principle of incident illumination has been developed for the irradiation of small parts of living cells or other microscopic objects. A reflecting microscope objective is used simultaneously in a visible light viewing‐aiming system and in a bombardment system to focus a spot of ultraviolet light (as small as one micron in diameter) onto part of an experimental preparation. Provision is made in the apparatus for auxiliary phase contrast viewing and for monitoring of the energy in the microbeam. Details of construction and alignment are given and the distribution of intensities in the microbeam is discussed.