It has become difficult to treat diseases effectively because microbial resistance to antimicrobials is spreading world over. The aim of this study was to understand the mechanism of resistance towards the antimicrobial Sulfamethaxazole by using resistant Enterobacter spp. and sensitive Enterobacter spp. There was complete inhibition of growth of sensitive Enterobacter spp. at 8 ?g/ml of Sulfamethaxazole, however, the resistant Enterobacter spp. tolerated even 24 ?g/ml of Sulfamethaxazole. Mechanism of resistance was found to be presence of plasmid. Plasmid isolation and analysis was done by agarose gel electrophoresis. Successful curing of plasmid was carried out with 10% sodium dodecyl sulfate (SDS). When colonies after SDS treatment were tested, resistant strains were found converted to sensitive ones. Importantly there was absence of b lactamase enzyme.