Tea tree oil (TTO), derived from the Australian native plant Melaleuca alternifolia (Myrtaceae), is becoming established as an alternative topical antimicrobial agent, with antibacterial, antiviral and antifungal properties ascertained both in vitro and in a growing number of clinical trials.1 Whilst organisms such as Staphylococcus aureus (including methicillin-resistant S. aureus) and Escherichia coli are relatively susceptible to the antimicrobial action of TTO and its components,1–3 with MICs of 0.5% and 0.25% respectively, one organism that appears to be more resistant is Pseudomonas aeruginosa. Broth microdilution MICs for P. aeruginosa range from 1% to 8%; however, only small numbers of strains have been tested with most sample sizes less than 5.4 An opportunistic pathogen that demonstrates a high and increasing level of multi-drug resistance,5,6P. aeruginosa remains a major causative agent of nosocomial infections. Given the current deficiency of published data on TTO susceptibility in the genus Pseudomonas, and the resistance to conventional antibiotics in P. aeruginosa, the objectives of this study were to elucidate a clear susceptibility profile for P. aeruginosa as well as Pseudomonas putida and Pseudomonas fluorescens