. However, they reported on Azospirillum brasilensense with other PGPR bacteria like Azotobacter, Azorhizobium, B. megatherium and P. fluorescens. As reported by Bitton and Marshall (1980) and Foster and Bowen (1982), repulsive forces between bacteria would have been weakened by neutralization of negative ions in the acidic pH resulting in co-aggregation. After a decline in co-aggregation at pH 7.0, the increase observed in pH 8.0 could be attributed to the tendency of P. fluorescens and B. subtilis to re-aggregate. Stationary phase ( Joe et al., 2009) or early stationary phase ( Richard et al., 2000) is the optimal growth phase for maximum co-aggregation in bacteria. Our results are also in conformity with above reports.