Ubiquitin signalling plays an indispensable role in plant immune signalling. Pathogen infection leads to rapid increases in the ubiquitinated proteome. Here, we identified ubiquitinated proteins and ubiquitination sites in Arabidopsis thaliana leaf tissue infected with the virulent pathogen Pseudomonas syringae pv. maculicola ES4326. Leaves of the Col-0 cultivar were pressure infiltrated with 2X107 cfu/ml of pathogen and samples harvested 24 hours post-inoculation.