Summary  The nucleotide sequence of the Aspergillus niger tryptophan C (trpC) gene was determined. Northern hybridization and S1-mapping experiments showed the presence of a 2.6 kb trpC poly(A)⁺ RNA with two very short (5 and 6 nucleotides) noncoding 5-regions. Comparison of the predicted amino acid sequence with that of trp gene proteins of pro- and eukaryotic organisms revealed three functional domains (G, C, F) in the A. niger TrpC protein which catalyse the glutarnine amidotransferase reaction (GAT), the indole-3-glycerol phosphate synthase reaction (IGPS) and the N-(5-phosphoribosyl) anthranilate isomerase reaction (PRAI), respectively. These domains are highly conserved and bordered by short areas showing less homology. Within the F domain of the trpC gene in A. niger, A. nidulans and Neurospora crassa, a region encoding 30 amino acids was found which is absent in the analogous genes of Saccharomyces cerevisiae and prokaryotic organisms. This region has features of a mutated in-phase intron.