Inoculation of pepper leaves, Capsicum annuum cv. Early Calwonder ECW 10R, with strains of Xanthomonas campestris led to an accumulation of the phenolic conjugates feruloyltyramine (FT) and p-coumaroyltyramine (CT) 24 h postinoculation in nonhost- and gene-for-gene-determined incompatible interactions with X. campestris pv. campestris and X. campestris pv. vesicatoria, respectively. In contrast, neither compound was detected in compatible interactions with X. campestris pv. vesicatoria. The accumulation of FT and CT was preceded by an increase in the extractable activity of tyrosine decarboxylase as well as increases in the transcription of genes encoding phenylalanine ammonia-lyase and tyramine hydroxycinnamoyl transferase. No such changes were detected in compatible interactions. Very rapid accumulation of FT and CT occurred (4 h postinoculation) in pepper in response to a X. campestris pv. campestris mutant carrying a deletion of the hrp gene cluster. In contrast, hrp mutants of X. campestris pv. vesicatoria failed to elicit the production of FT and CT. These observations suggest the existence of hrp gene-dependent and -independent activation mechanisms of a defense response involving hydroxycinnamoyltyramines.