Effects of short-chain primary amines on Aspergillus nidulans development were analyzed. Propylamine induced asexual development and inhibited sexual development. Even on medium containing lactose as the sole carbon source, on which little conidial heads are formed and sexual structures are formed preferentially, or when sexual development was induced, propylamine induced asexual development and inhibited sexual development. These effects of propylamine seemed to be due to accumulation of mRNA of the brlA gene, which has been identified as a positive regulator of asexual development, and due to the reduction of the veA mRNA level. The veA gene has been identified as an activator of sexual development and also as an inhibitor of asexual development. Other primary amines, methylamine and ethylamine, showed identical effects on development where short-chain primary amine also promoted asexual development and inhibited sexual development.