The diameter of inhibition zones in cultures of Ps. aeruginosa produced by discs of gentamicin varied on different media, as did the minimum inhibitory concentration (MIC) when the antibiotic was included in the medium. These variations, which were small in parallel tests of Esch. coli, were found to depend on the general composition of the medium, but were also affected by the agar used to solidify it. An agar on which the MIC was low had a low magnesium content, and two on which it was high had a high content. The addition of magnesium sulphate in increasing amounts to a medium of low content progressively increased the MIC. It thus appears that a high magnesium content, which is necessary for pyocyanin production, also increases the resistance of this organism to gentamicin.The difficulty of determining the sensitivity of Ps. aeruginosa to gentamicin as a guide to treatment is emphasized. The best method appears to be direct comparison with another strain of the same organism of known sensitivity.