The authors have studied on 20 bacterial strains (5 Pseudomonas aeruginosa, 5 Escherichia coli, 5 Klebsiella and 5 Serratia) the bactericidal kinetics of 4 aminoglycoside antibiotics: amikacin, gentamicin, kanamycin and tobramycin. The antibiotic concentrations used for this work were 1.5 times the MIC for each strain previously measured in a liquid medium. The action of the aminoglycoside antibiotics shows three phases. The third phase describes a part of less susceptible bacterial population. It permits the comparison of the 4 antibiotics. Amikacin shows the best activity, followed by gentamicin, kanamycin and tobramycin.