High concentration of inorganic phosphate in the culture medium of Aspergillus fumigatus inhibited ergot alkaloid synthesis. Addition of L-tryptophan but not mevalonate or 5-methyltryptophan to the above culture restored the alkaloid synthesis to the level found in normal cultures. The decrease in alkaloid synthesis in the fungus accompanies an increase in cell mass, cellular protein and sterol content. Aspartate aminotransferase and alanine aminotransferase activities were significantly increased in the high-phosphate culture.