A tobacco (Nicotiana tabacum L. cv Samsun NN) cDNA clone coding the enzyme phenylalanine ammonia-lyase (PAL) was isolated from a cDNA library made from polyadenylated RNA purified from tobacco mosaic virus (TMV)-infected leaves. Southern analysis indicated that, in tobacco, PAL is encoded by a small family of two to four unclustered genes. Northern analysis showed that PAL genes are weakly expressed under normal physiological conditions, they are moderately and transiently expressed after wounding, but they are strongly induced during the hypersensitive reaction to TMV or to a fungal elicitor. Ribonuclease protection experiments confirmed this evidence and showed the occurrence of two highly homologous PAL messengers originating from a single gene or from two tightly co-regulated genes. By in situ RNA-RNA hybridization PAL transcripts were shown to accumulate in a narrow zone of leaf tissue surrounding necrotic lesions caused by TMV infection or treatment with the fungal elicitor. In this zone, no cell specificity was observed and there was a decreasing gradient of labeling from the edge of necrosis. Some labeling was also found in various cell types of young, healthy stems and was shown to accumulate in large amounts in the same cell types after the deposition of an elicitor solution at the top of the decapitated plant.