The avirulence gene avrXa10 of Xanthomonas oryzae pv oryzae directs the elicitation of resistance in a gene-for-gene manner in rice lines carrying the resistance gene Xa10. We have localized a transcriptional activator domain in the C terminus of AvrXa10 by using amino acid replacement mutagenesis. One mutant, with replacements at three hydrophobic amino acid residues in the C-terminal domain, was defective for transcriptional activation in yeast and avirulence activity in rice. The activation domain from the herpes virus protein VP16 restored the ability of the bacteria expressing the hybrid protein to elicit a resistance reaction. Elicitation was specific for Xa10, and the reaction had the hallmarks of the response to AvrXa10. The results indicate that a domain with the properties of a transcriptional activator plays a critical role in AvrXa10 function. The results also indicate that the protein has the potential to interact with the plant transcriptional program, although a role for the domain in the stability or conformation of the protein in the plant cannot be excluded. In a broader sense, the transcriptional activation domain of avrXa10 may represent a prokaryotic version of the acidic transcriptional activation domain, which heretofore has been found exclusively in eukaryotes.