The stress-induced expression of four different ubiquitin-encoding cDNAs was characterized in potato tuber tissue. The four clones exhibited differences in both structure and expression. The first cDNA encoded a single ubiquitin unit fused to an 80 amino acid ribosomal extension protein identical to the extension protein from tomato. Accumulation of the fusion transcript was induced by injury or ethylene, but not by heat shock. The three remaining ubiquitin cDNAs encoded polyubiquitins with 6 to 7 ubiquitin repeats. The first polyubiquitin gene was induced by injury, heat, or ethylene treatments. The second was induced also by injury or heat, with limited ethylene-dependent accumulation of transcript. Transcript levels of the third polyubiquitin gene were highest in control tubers and decreased markedly with injury, heat shock, or ethylene treatment. The data demonstrate the independent regulation of the different members of the ubiquitin gene family in response to stress and exogenous ethylene.