We conduced a cross-sectional study to determine the prevalence of resistant mycobacteria in our setting. All patients in whom M. tuberculosis had been isolated in cultures of clinical samples (119) between January 1992 and December 1993 took part in the study. Canetti's method of percentages was used for the study of sensitivity to the following drugs: isocyanide, rifampicin, streptomycin, ethambutol and para-aminosalicylic acid. Overall resistance of M. tuberculosis was 9.24%. Specifically, we found resistance to isocyanide in 5.9%, to streptomycin in 4.20%, to rifampicin in 5.04% and to ethambutol in 1.68%. Resistance was primary in 7% and secondary in 21.05%; 2% showed primary resistance to isocyanide, 1% to isocyanide and rifampicin, 2% to streptomycin, 1% to ethambutol and 1% to streptomycin and rifampicin. These resistance indices are in keeping with those published for other areas of Spain.