The influence of bioreactor operational conditions on the micromorphology of batch cultures of Aspergillus niger B1-D, containing a hen egg white lysozome (HEWL) marker protein, was examined using computerised image analysis. Significant differences in micromorphology were observed with increased stirrer speed, with shorter organisms with shorter hyphal elements occurring as agitation speed increased, even though mean tip numbers were similar. This may explain the observed increase in the total extracellular protein, since the ratio of synthetic (tip) to non-synthetic zones became increasingly favourable. HEWL concentrations fell above 500 rpm, probably due to the effects of DOT (dissolved oxygen tension) on the glucoamylase HEWL fusion. HEWL was susceptible to proteolytic degradation, by native proteases, during the autolytic phase. Such insights may indicate why a gene from one mould expressed in a close relative can give production levels equivalent to levels of native enzymes, while secretion of a gene from a 'distant' source, eg a higher eukaryotic gene, occurs at much lower levels.