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Intracellular targeting of the Pseudomonas aeruginosa toxins exoenzyme S (ExoS) and exoenzyme T (ExoT) initially results in disruption of the actin microfilament structure of eukaryotic cells. ExoS and ExoT are bifunctional cytotoxins, with N-terminal GTPase-activating protein (GAP) and C-terminal ADP-ribosyltransferase activities. We show that ExoS can modify multiple GTPases of the Ras superfamily in io. In contrast, ExoT shows no ADP-ribosylation activity towards any of the GTPases tested in io. We further examined ExoS targets in io and observed that ExoS modulates the activity of several of thes