Effective mixing of a cylindrical bath is of crucial for wastewater treatment. Measurements were carried out on the mixing time in a cylindrical bath agitated simultaneously by bottom gas injection and side liquid injection. Air was injected into the bath through a centered single-hole bottom nozzle. Water was injected into the bath through a single-hole nozzle attached to the side wall of the vessel, drained from a pipe installed at the opposite bottom corner, and injected again with a small pump. The mixing time due to simultaneous gas and water injection, T m3 , was satisfactorily correlated by the following empirical equation.where T m1 and T m2 are the mixing time values for water injection alone and gas injection alone, respectively. This relationship means that mixing by gas injection and that of liquid injection proceed in parallel with each other.