The growth and metabolism of a normal strain (A 69) of Aspergillus nidulans upon a simple chemically defined medium has been followed analytically. The main metabolic products were mycelial material, which reached its maximum after about 8 days, and CO 2 Negligible quantities of small-molecule carbon-compounds were detected. A minute amount of volatile acid which was maximal at the time of maximal felt-weight was found. Nitrate, the sole nitrogen-source, was all utilized. The felt took up most of the nitrogen and retained it in spite of the decrease in mycelial matter due to senescence. Negligible amounts of volatile-base nitrogen were formed. Sulphate was partly assimilated by the mould. No non-sulphate-sulphur compounds could be detected in the liquor. Like the mycelial nitrogen, once taken up the sulphur remained in the felt throughout the period of the experiments (20 days).