Production of extracellular galactose oxidase by Fusarium sp. NRRL 2903 appears to be best in medium containing L-sorbose, which acts as a paramorphogen for several filamentous fungal species. Analysis of galactose oxidase activity in culture supernatants and the occurrence of gaoA mRNA, during growth in media containing either L-sorbose or glycerol as the sole source of carbon, indicates that the two carbon sources result in high and low levels of extracellular galactose oxidase activity, respectively, which is in parallel with the level of transcription. The influence of altering the carbon source for growth on the regulation of galactose oxidase synthesis and secretion appears to be mainly at the level of gene expression rather than pH or cell density, although these factors are also known to have some regulatory effect on the level of secretion. Kinetics of galactose oxidase secretion, evaluated on the basis of altering the pH of the growth medium, agrees with the Luedeking-Piret equation and suggests that the growth associated and the non-growth associated product formation rates are changed in expense of each other.