The isolation and characterization of a genomic clone encoding proteinase inhibitor II of potato (Solanum tuberosum) is described. The structure of this gene was determined by sequencing a genomic fragment of about 2 kb containing the entire RNA coding as well as about 900 nucleotides of the 5'-upstream and 250 nucleotides of the 3'-downstream region. The transcription start site was determined by RNase protection experiments. The comparison of the genomic sequence with cDNA sequences reveals the presence of one intron with a length of 117 nucleotides. The genomic clone contains an open reading frame of 462 nucleotides allowing for a protein of 154 amino acids. The proteinase inhibitor II gene displays typical features of eucaryotic genes. The sequence TATAAA is found 26 nucleotides upstream of the transcription initiation site and the sequence CAAAT at position--103. In the 3'-region the sequence AATAA is found 33 nucleotides in front of the poly-A addition site.