The essential oil of Melaleuca alternifolia (tea tree oil) and its components have antimicrobial activity against a wide range of Gram-positive and Gram-negative bacteria, fungi and viruses. The mechanism(s) by which Pseudomonas aeruginosa NCTC 10662 maintains a decreased susceptibility to M. alternifolia oil and components was investigated. Ethylene diamine tetraacetic acid enhanced the antimicrobial activity of M. alternifolia oil and terpinen-4-ol against stationary phase P. aeruginosa while polymyxin B nonapeptide enhanced the activity of M. alternifolia oil and gamma-terpinene. Pretreatment with the protonophore carbonyl cyanide m-chlorophenylhydrazone increased the susceptibility of exponential phase cells to sub-inhibitory concentrations of M. alternifolia oil, terpinen-4-ol and gamma-terpinene, indicating that intrinsic tolerance to M. alternifolia and components is substantially energy dependent.