The process of cellular autolysis was studied in an industrial strain of Penicillium chrysogenum by a range of methods, including assessment of biomass decline, NH 4 + release, changes in culture apparent viscosity, and by means of a quantitative assessment of changes in micromorphology using a computerized image analysis system. The pattern of total intracellular proteolytic and β€-1,3-glucanolytic activity in the culture was also examined. The overall aim was to identify a suitable method, or methods, for examining the extent of autolysis in fungal cultures. Autolysis was studied in submerged batch processes, where DOT was maintained above 40% saturation (non-O 2 -limited), and, under O 2 -limited conditions. Both N and O 2 limitation promoted extensive culture autolysis. Image analysis techniques were perhaps the most sensitive method of assessing the progress of autolysis in the culture. Autolytic regions within some hyphae were apparent even during growth phase, but became much more widespread as the process proceeded. The early stages of autolysis involved continued energy source consumption, increased carbon dioxide evolution rate, degradation of penicillin, and decreased broth filterability. Later stages involved widespread mycelial fragmentation, with some regrowth (cryptic growth) occurring in non-O 2 -limited cultures. Intracellular proteolytic activity showed two peaks, one during the growth phase, and the other during autolysis. Autolysis was also associated with a distinct peak in β€-1,3glucanolytic activity, indicating that degradation of cell wall matrix polymers may be occurring during autolysis in this strain of P. chrysogenum.