Potato (Solanum tuberosum L. cv. Datura) contains approximately 40-50 phenylalanine ammonia-lyase (PAL) genes/haploid genome. Considerable cDNA heterogeneity indicates that at least about 10, and probably more, of these genes are potentially active. One subfamily, represented by one selected member (PAL-1), was analyzed with respect to genomic complexity, nucleotide and deduced amino acid sequence, and mode of constitutive or induced expression. For comparison, a second gene (PAL-2), representing several subfamilies that are easily distinguished from PAL-1, was included in these studies. Extensive structural similarities were observed both between the TATA-proximal portions of the PAL-1 and PAL-2 promoters, particularly in the areas containing putative cis-acting elements, and among all presently known PAL proteins from various higher and lower plants. The relative abundance of PAL mRNA varied greatly in several major potato organs. However, the patterns obtained with probes detecting either total PAL mRNA or more specifically, PAL-1-related or PAL-2-related mRNA species, were the same within experimental error. Mature leaves contained particularly low levels of PAL mRNA. Infection of these leaves with the pathogenic fungus, Phytophthora infestans, resulted in a large, transient induction of PAL mRNA. The relative timing of PAL-1 and PAL-2 mRNA expression, however, differed in compatible (fungus virulent, plant susceptible) but not in incompatible interactions (fungus avirulent, plant resistant). Wounding of leaves caused an extremely rapid and transient induction of both PAL mRNA species.