This paper describes a sensitive, automatic recording microspectrophotometer capable of measuring the light absorption of substances present at ``enzyme'' concentrations in a single living cell. Modified dynode‐feedback and measuring circuits have been especially adapted for measurement of small photocurrents at relatively high signal‐to‐noise ratios. The application of the instrument to observations of the mitochondrial aggregate (Nebenkern) of the grasshopper spermatid is discussed.