A method is described for the assessment of the metabolic activity of the filamentous fungus Acremonium chrysogenum using the fluorescent dye Acridine Orange. Changes in metabolic activity are indicated by a reversible red-green shift in the colour of the dye, quantifiable by image analysis. A. chrysogenum mycelia exhibited an overwhelmingly green colour in circumstances leading to high substrate uptake, while under carbon starvation they appeared orange-red. It is believed these colour changes reflected changes in internal pH. The method provides a visual tool for the investigation of the metabolic behaviour of filamentous fungi.