We have cloned the trifunctional trpC gene from Aspergillus nidulans by hybrid phage lambda complementation of an Escherichia coli trpC mutant lacking phosphoribosylanthranilate isomerase activity. Four different phages sharing a 4.3-kilobase region were obtained. Plasmid subclones containing this region also complemented the E. coli trpC mutant. We determined that a 1.8-kilobase DNA fragment was minimally required for complementation. The fragment hybridized with two poly(A)+ RNAs, 3.0 and 3.2 kilobases in length. We infer that these transcripts are Aspergillus trpC mRNAs and that the entire Aspergillus trpC gene is not required for complementation in E. coli. Levels of both trpC transcripts in poly(A)+ RNA are regulated by growth medium composition. They were highest when cells were grown in minimal medium containing nitrate as the nitrogen source and lowest when cells were grown in medium containing yeast extract. The concentrations of the transcripts are also regulated during conidiophore development. Conidiating cultures grown on medium containing yeast extract had significantly higher levels of both transcripts than did hyphae grown in minimal medium containing nitrate. Levels of the transcripts in mature spores were equivalent to those found in hyphae grown in minimal medium containing nitrate. Results from nutritional experiments with an A. nidulans trpC mutant suggest that developmental regulation of trpC mRNA levels may be related to a high requirement for tryptophan or a compound derived from tryptophan during conidiation.