Two closely linked, highly homologous phenylalanine ammonia-lyase (PAL) genes located in one phage clone, PAL1 and PAL2, were isolated from a wheat genomic library by using a polymerase chain reaction (PCR) fragment of a wheat PAL gene as probe. The two PAL genes were located approximately from 7 kb apart and displayed 93% identity with the same orientation. Southern blot analysis with a PAL1 specific fragment as probe showed the presence of a multiple gene family of PAL in wheat. Northern analyses demonstrated a differential expression of PAL in two Chinese Spring near-isogenic lines upon infection with stem rust fungus Puccinia graminis. In the resistant Chinese Spring Sr11 isogenic line with the resistance gene Sr11 that is known to interact with the avirulence gene P11 from the stem rust fungus, an induced expression of PAL was observed 4 days post inoculation (d.p.i.), and a massive induction was evident 8 d.p.i. By contrast, in the susceptible Chinese Spring sr11 line lacking the resistance gene, induction of PAL was seen 6 d.p.i. and the expression level at 8 d.p.i. was similar to that observed in the resistant line at 6 d.p.i. In wheat suspension culture cells, treatment with either an elicitor isolated from the stem rust fungus or chitin oligomers could activate PAL gene expression within 2 h. The fungal elicitor appeared to be more active in early activation of the PAL gene than were chitin oligomers. These results demonstrated that wheat PAL played an important role in the induced resistance response upon infection with stem rust fungus at transcription level.