The nucleotide sequence of a 2994-bp genomic fragment, including the gamma -actin encoding gene from Penicillium chrysogenum, has been determined, showing an open reading frame (ORF) of 1756 bp interrupted by five introns with fungal consensus splice-site junctions. The 5' untranslated region contains a consensus TATA box, five CAAT motifs, and two large pyrimidine stretches. The predicted protein (375 amino acids) revealed high identity to gamma -actins from fungi (>90%), and gene phylogenies support the grouping of P. chrysogenum actin close to those from the majority of the filamentous fungi. The actA gene is present as a single copy in the genome of P. chrysogenum, and its expression is constitutive during penicillin fermentation, showing a single 1.4-kb transcript.