Summary Twelve Erwinia chrysanthemi genes specifically up-regulated during infection in chicory leaves have been identified using GUS reporter gene fusions. According to sequence comparisons with GenBank databases, the possible functions of the identified genes included: virulence, adaptation to apoplast environment, chemotaxis, transposition, regulation, and metabolic functions. The level of gene up-regulation in planta was measured, and ranged from 1.39 to 47.11 times that found in liquid culture. This technique has been proven to be useful in the identification of bacterial up-regulated genes in plant-microbe interactions.