Figure 2a shows the biosurfactant production Kinetics from D. caeni PO5 using the optimal medium and cultivation conditions reported above. Biosurfactant production and surface tension were dependent on the growth of the culture in the fermentation medium. The surface tension dropped rapidly after 18 h of cultivation, reaching its lowest value (25.0 mN/m) after 63 h of cultivation. Howevwe, the highest biosurfactant yield (3.12 g/l) was obtained as the cultivation tine approached 87 h, which also corresponded to the stationary phase of microbial growth. These reslts lead to the conclusion that the biosurfactant produced by D. caeni PO5 is a primary metabolite. Growth-associated production of biosurfactant has been reported for Pseudomonas sp. (Obayoi et al. 2009), Selenomonas ruminantium CT2 (Saimmai et al. 2012a), Oleomonassagaranensis AT18 (Saimmai et al. 2012C), and Bacillus spp. (Suwansukho et al. 2008). From the results it can be seen that a cultivation time of 87 h gave the highest biosurfactant activity.