The present paper deals with the study of the growth of Xanthomonas campestris (Pammel) Dowson sensitive to streptomycin, penicillin and aureomycin, compared with the growth of three resistant mutants to these drugs. The original strain sensitive, presented the same growth rate than the resitant strains to streptomycin and penicillin but, grew better than the resistant strain to aureomycin. The resistant mutants to streptomycin and aureomycin presented the same growth rate in liquid medium plus antibiotic and in liquid medium without antibiotic. The resistant strain to penicillin grew better in medium without this antibiotic. The ratio between sensitive cells and resistant cells were studied when a mixture of cells were made.