The proline catabolism gene cluster of Aspergillus nidulans was cloned using a 'brute force' technique which detects clones hybridizing to restriction fragments overlapping chromosomal rearrangements. A number of deletion mutations and a translocation mutation in the cluster have been physically mapped, and an excellent correlation between the genetic and physical maps was established. Transcripts have been identified and orientated for each of the four genes of the cluster. All are monocistronic by size. All of the transcripts, including that of the regulatory gene prnA, are inducible. Using deletion endpoints and mRNA sizes, approximate gene positions on the physical map have been determined. Finally, the relationship between genetic and physical distance across the cluster has been estimated at 3-4 kilobases per centiMorgan.