Although manganese had been shown to be an essential requirement for patulin biosynthesis, its site of action was unknown. Four possibilities were considered. A manganese requirement for the second pathway enzyme, a decarboxylase, was discounted since mid and late pathway intermediates were not converted to patulin in manganese-deficient cultures. A major disruption in primary metabolism and hence secondary metabolism was discounted since eight primary metabolism enzymes showed no evidence of unusual changes in specific activity when normal and manganese-deficient cultures were compared. Inhibitor studies using actinomycin D and cycloheximide showed that there was no activation of preexisting enzyme proteins and that manganese did not influence translation. The inhibitor studies did show, however, that manganese exercised its effect on patulin biosynthesis by influencing the coordinate appearance of pathway enzymes though an effect at the level of transcription.