Sensitivity of 200 strains of Pseudomonas aeruginosa to 8 antibiotics was studied with 2 methods, agar dilution and agar diffusion. The data obtained with the two methods were in good agreement. The simple method of agar diffusion provided sufficiently precise results in determination of the Pseudomonas sensitivity to carbenicillin and polymyxin. With the use of the correction principle by the "mobile intermediate zone" it also provided sufficiently precise results in determination of the Pseudomonas sensitivity to gentamicin. The necessity of using the reference strain ATCC 27853 of P. aeruginosa in every experiment is stressed. The peculiarities of the data interpretation in determination of antibiotic sensitivity of Pseudomonas are discussed.