Twenty-five species were isolated from oil and polluted soil samples of Dorood oilfield, in the south of Iran, which were able to utilize asphaltene as a sole carbon and energy sources. Isolated microorganisms, in the form of pure and mixed, were cultured for 2 months at 28 degrees C in a medium containing asphaltene. Five of the isolated microorganisms which higher growth on asphaltene were identified as Pseudomonas spp TMU2-5, Bacillus licheniformis TMU1-1, Bacillus lentus TMU5-2, Bacillus cereus TMU8-2, and Bacillus firmus TMU6-2. These bacteria degrade 40-46% of asphaltene and among them B. lentus TMU5-2 showed the greatest capability for degradation asphaltene. Biodegradation of asphaltene by the mixed culture of five bacteria was 48%, this is the highest reported degradation result. The bacterial mixed culture growth, in a bubble-column (55 x 7.5 cm), was evaluated by CHN and FTIR analysis during 2 months of incubation. The values of pH, salinity and asphaltene concentration for degradation at 40 degrees C were optimized by response surface methodology for pure cultures of B. lentus and the optimum results are 6.7, 76 g L (1) and 22 g L (1), respectively, and for the mixed culture as 6.4, 76 g L (1) and 12 g L (1), respectively. Conducted kinetic study showed that the data of asphaltene biodegradation fit to Tessier model and the growth kinetic parameter were obtained as mu(max) = 0.31 day (1), k(s) = 39.19 g L (1) and Y-xs = 0.21. (C) 2011 Elsevier Ltd. All rights reserved.