The nucleotide sequences of regions containing the mating-type locus of the plant-pathogenic ascomycete Leptosphaeria maculans are described. The MAT1-1 gene is 1,368 bp, encoding a predicted protein of 441 amino acids, with a 45-bp intron. The MAT1-2 gene is 1,246 bp, encoding a predicted protein of 397 amino acids, with a 55-bp intron. This latter gene is 334 bp downstream of a small open reading frame (32 amino acids) with four amino acids in identical positions to those in the high mobility group binding domain of the MAT1-2 genes. The DNA lyase and anaphase promoting complex genes are 3' of the MAT gene, whilst a gene denoted ORF1 in Cochliobolus heterostrophus and the GTPase activating protein are present 5' of MAT. The transcriptional patterns of genes within and flanking the L. maculans MAT locus are determined. The MAT transcripts are about twice the length of the gene. The ORF1 transcript is 1.2 kb in the MAT1-1 isolate and 1.0 kb in the MAT1-2 isolate; and probes cross-hybridise weakly. A mating-type PCR assay with three nucleotide primers is developed for L. maculans.