A variety of assays were utilized to determine the effects of Leptospermum petersonii essential oil on both Candida albicans and Aspergillus fumigatus. Hyphal morphology, susceptibility of spheroplasts and uptake of propidium iodide following exposure to the oil suggest that the mode of action of L. petersonii essential oil is through direct disturbance of the fungal cell membrane. Data also confirms that the volatile component of the oil is highly antifungal, independent of direct contact between the liquid oil and the fungal membrane. The degree of inhibition was greater when fungi were directly exposed to oil volatiles compared to pre-inoculation exposure of oil volatiles into the agar. It is likely that the essential oil volatiles are acting both directly and indirectly on the fungi to produce growth inhibition.