Pseudomonas aeruginosa, an opportunistic pathogen, is capable of establishing both chronic and acute infections in compromised hosts. Previous studies indicated that P. aeruginosa displays either a cytotoxic or an invasive phenotype in corneal epithelial cells. In this study, we used polarized MDCK cells for in vitro infection studies and confirmed that P. aeruginosa isolates can be broadly differentiated into two groups, expressing either a cytotoxic or an invasive phenotype. In vivo infection studies were performed to determine if cytotoxic and invasive strains displayed differential pathology. Invasion was assayed in vivo by in situ infection of mouse tracheal tissue followed by electron microscopy. Both cytotoxic and invasive strains entered mouse tracheal cells in situ; however, more necrosis was associated with the cytotoxic strain. In an acute lung infection model in rats, cytotoxic strains were found to damage lung epithelium more than invasive strains during the short infection period of this assay. The expression of cytotoxicity requires a functional exsA allele. In the strains tested, the ability to invade epithelial cells in vitro appears to be independent of exsA expression. Since ExsA is a transcriptional regulator of the exoenzyme S regulon, chromosomal preparations from invasive and cytotoxic strains were screened for their complement of exoenzyme S structural genes, exoS, encoding the 49-kDa ADP-ribosyltransferase (ExoS), and exoT, encoding the 53-kDa form of the enzyme (Exo53). Invasive strains possess both exoS and exoT, while cytotoxic strains appear to have lost exoS and retained exoT. These data indicate that the expression of cytotoxicity may be linked to the expression of Exo53, deletion of exoS and perhaps other linked loci, or expression of other ExsA-dependent virulence determinants. In the absence of a functional cytotoxicity pathway (exsA::omega strains), invasion of eukaryotic cells is detectable.