The process of autolysis in batch cultures of an industrial strain of Penicillium chrysogenum was investigated at a range of stirrer speeds. Autolysis in the cultures was monitored by conventional analysis (biomass decline, NH4+ release), direct measurement of autolysing regions (image analysis), and enzyme activity assays (proteases and beta-glucanases). Image analysis measurements provided a sensitive indicator of the onset and progress of autolysis in the culture. Autolysis could be brought about by C or N limitation. Culture history could have a marked effect on the process. Autolysis resulted in degradation of penicillin V and problems with broth filterability. It was closely associated with increased intracellular proteolytic and beta-1-3-glucanolytic activities. Although stirrer speed affected both growth and penicillin V production, final mean main hyphal lengths were similar in autolysing cultures. This indicated a possible role of an intrinsic characteristic in fragmentation.