Six diesel-degrading bacterial strains were isolated from oil-polluted sites located in central Taiwan. The floating activity of the isolates in an oil-supplemented liquid medium was monitored. Cell-surface hydrophobicity as well as cell-free and cell-residue emulsification activities were also investigated. Three isolates, identified as Gordonia alkanivorans CC-JG39, Rhodococcus erythropolis CC-BC04, and R. erythropolis CC-BC11, were found to float and grow near the diesel layer on the surface. The other three isolates (namely, Comamonas testosteroni CC-CF3, Acinetobacter sp. CC-CF5, and Sphingomonas yanoikuyae CC-CG22) did not display floating activity, as they distributed uniformly in the liquid medium. Isolated cell walls of the floating strains appeared to settle at a lower sucrose density than the nonfloating strains. The floating strains were also characterized by a higher cell-surface hydrophobicity and a higher cell-residue emulsification activity than the nonfloating strains. In fact, the floating strains were thought to produce extracellular emulsifiers due to their higher supernatant emulsification activity than the nonfloating strains. The floating activity of G alkanivorans CC-JG39 may be associated with the production of extracellular polymeric substances that formed an "air-bag" structure facilitating cell floating. The floating ability may also correlate with a high cellular hydrophobicity arising from unique cell wall compositions or cell-wall-bound surface active products.