A defined medium containing glucose, mineral salts; and the amino acid L-proline was developed for the propagation of Penicillium clavigerum. Certain modifications of this medium produce dramatic differences in colonial morphology. Coremia are produced only if certain dipeptides are added to the medium. The dipeptides shown to produce this response were DL-alpha aminobutyric acid-DL-leucine, DL-leucyl-DL-alanine, DL-alanylglycine, and glycyl-DLaspartic acid. No sporophores were produced in the absence of these dipeptides, nor was dry weight affected. The fungus does not require vitamins for growth, but the addition of folic acid to the medium increased the quantity of spores and coremia.