A microspectrophotometer of increased sensitivity has been constructed to rapidly record absorption spectra in a single sweep at relatively low light levels from the ultraviolet through the visible, from 2500 to 8000 Å. The visible spectrum can be scanned within 2 sec. Absorption spectra from single living cells, cell organelles and particles as small as 0.5 μ diam can be obtained. The design of the optical system, the electronics and properties of the components of the instrument are given. Examples of its performance are presented and its versatility for obtaining spectral data is discussed.