Comparable results were obtained by Kim et al. (1997). They found that the surface tension reducing activity of Bacillus subtillis C9 was stable to pH over the range of pH 5.0 to 9.5. Also, Abu-Ruwida et al. (1991), observed that the biosurfactant production of Rhodococcus was at pH 6.5 to 7.2 that determined by surface tension