Aims: To investigate antifungal effect of thyme oil on Geotrichum citri-aurantii arthroconidia germination and germ tube elongation, to reveal effects of thyme oil on morphological structures on fungal hyphae and arthroconidia and to assess potential bio-control capacities of thyme oil against disease suppression in vivo conditions. Methods and Results: Thyme oil controlled the growth of G. citri-aurantii effectively. Arthroconidia germination and germ tube elongation in potato dextrose broth was greatly inhibited by thyme oil. At 600 mu l l-1, it inhibited the germination of about 94% of the arthroconidia and the germ tube length was only 4 center dot 32 +/- 0 center dot 28 mu m. Observations using light microscope, scanning electron microscope and transmission electron microscope revealed ultrastructural modifications caused by thyme oil that included markedly shrivelled and crinkled hyphae and arthroconidia, plasma membrane disruption and mitochondrial disorganization. Thyme oil applied to 'Satsuma' mandarin oranges that had been artificially wounded and inoculated with G. citri-aurantii reduced sour rot from 78 center dot 1% among untreated control fruit to 14 center dot 1% after 5 days at 26 degrees C. Thyme oil applied to intact fruits reduced the decay from 76% among untreated control fruit to 35% after 30 days at 20 degrees C. Thyme oil treatment did not harm 'Satsuma' mandarin oranges when they were examined after treatment and storage at 20 degrees C for 30 days. Conclusions: Thyme oil may provide an alternative means of controlling postharvest sour rot on citrus fruit. Significance and Impact of the Study: The use of such essential oil may constitute an important alternative to synthetic fungicides. They can be exploited in commercial production and applied under storage and greenhouse conditions.