Aims: In order to add to the existing knowledge about structural diversity of biosurfactants, marine environment was chosen to discover a new type of biosurfactant-producing fungus. Methods and Results: A number of fungi were collected from the Gulf of Thailand and examined for biosurfactant productivities. A dimorphic fungus, Aureobasidium pullulans YTP6-14, produced several different biosurfactants in both heavy oil and aqueous layers of the culture. Surface tension of the aqueous layer was decreased to 31.4 mN m(-1) and oil displacement area reached 53 cm(2)/10 mu l after 7 days of cultivation. Critical micelle concentration and minimum surface tension values of the crude biosurfactants prepared from the aqueous layer were 39 mg l(-1) and 31.6 mN m(-1) respectively. Surface tension values remained unchanged over a wide range of pII and NaCl concentrations, suggesting their nonionic feature. LC/MS and NMR analyses revealed that one of the main active compounds in the aqueous layer was 5-hydroxy-2-decenoic acid delta-lactone, known as massoia lactone. Massoia lactone indeed showed significant surface tension reduction capacity of 43.3 mN m(-1) at 1 mg ml(-1). Significance and Impact of the Study: This is the first report for the production of a fragrant biosurfactant, massoia lactone by a fungus A. pullulans. Massoia lactone has been industrially prepared from aromatic bark of an endangered tree species, Cryptocarya massoy, growing in rainforests. This report expands the diversity of biosurfactants produced by A. pullulans and also points to its possibility in contributing to the green sustainable chemistry, and ultimately rainforest conservation.