The invitroresistance of 40 strains ofStaphylococcus aureusisolated from dogs to trimethoprim, sulphamethoxazole and co‐trimoxazole (SXT) was assessed using the disc diffusion technique on Diagnostic Sensitivity Test Agar (DSTA), DSTA with 5% lysed horse blood (DSTA + B), IsoSensitest Agar (ISTA) and Wellcotest Sensitivity Test Agar (WSTA).When sensitivity was based on a zone width of ≥4 mm all 40 strains were sensitive to SXT on DSTA + B and ISTA, 39 were sensitive on WSTA while only 31 (77.5%) were sensitive on DSTA.A comparison of the results obtained for each strain for trimethoprim and sulphamethoxazole with those obtained with the Oxford strain ofS. aureusand using a scheme for interpretation recommended by Garrod&O'Grady (1971) indicated that the proportion of strains classed as sensitive for therapeutic purposes was 5%, 55%, 47.5% and 15% respectively on DSTA, DSTA + B, ISTA and W