The production of gamma -decalactone by the filamentous fungus Trichoderma harzianum involves four phases (oil-water-air-mycelium) and its dispersion is crucial during fermentation. Oil and biomass (when present) dispersion, as a function of the volumetric power drawn (P/V), was characterized, in two; three-and four-phase systems agitated with Rushton turbines. Trichoderma harzianum mycelium was used as the solid phase in the four-phase system. Two stages of the fermentation were simulated: the beginning (15% oil and 1.4kgm(-3) of mycelium) and the end (2% oil and 10.6kgm(-3) of mycelium). In the two-phase system, the use of exhausted broth achieved higher oil dispersions at low PIV values as compared with distilled water. Aeration decreased the oil dispersion for the high-oil system, but enhanced oil dispersion for the low-oil system. Compared with the P/V used in the actual fermentation (0.2 kWm(-3)), a high segregation of the system was observed for the high-oil/low-biomass system, due to the difficulty of mixing the thick oil-air emulsion present at the top of the tank. The system simulating the end of the fermentation reached almost complete homogeneity of oil a nd biomass, a phenomenon due to the high biomass/oil ratio: and the biomass acting as an oil carrier.