03to nalidixic acid by mutation in gyrA gene which replace nalidixic acid from the target site in gyrase A 5,3 . These nalidixic acid resistant Salmonella are also less responsive to ciprofloxacin due to less hydrogen bonding formed between ciprofloxacin and altered gyrase A. Resistance of Salmonella to nalidixic acid is mediated by alteration of amino acid in DNA gyrase which is result of mutation in gyrA and gyrB chromosomal genes and by alteration of amino acid in tropoisomerase which is result of mutation in parE and perC genes or by decreased permeability of drug or over-expression of efflux pump. However, mutation in gyrA gene is more common than gyrB, parE and perC genes 6 . Resistance mutations in gyrA gene is clustered in a region of gene product 67-106 amino acids called quinolone resistancedetermining region and most common mutations associated with quinolone resistance are located in amino acid, serine-83 or aspartic acid-87 7 . Single mutation in gyrA gene (either amino acid serine-83 or aspartic acid-87) is associated with resistance to nalidixic acid and decreased susceptibility to INTRODUCTION Enteric fever (typhoid and paratyphoid fever) is an endemic disease in developing countries 1 . The etiological agent of typhoid fever is Salmonella typhi and paratyphoid fevers are Salmonella paratyphi A, B and C 2 . Among these Salmonella, Salmonella typhi and Salmonella paratyphi A are mainly responsible for enteric fever. Antibiotic is the main therapeutic option for the treatment of enteric fever and mortality rate may reach up to 30% in the absence of effective antibiotic therapy 3 . Since Salmonella are resistant to conventional antibiotics (ampicillin, chloramphenicol and cotrimoxazole), ciprofloxacin and ofloxacin had been the drugs of choice in Bangladesh4. But rampant use of ciprofloxacin made Salmonella typhi and Salmonella paratyphi A resistant Total 323 blood samples were collected from suspected enteric fever patients from medicine and paediatric units of RMC hospital and cultured on brain heart infusion broth for isolation of Salmonella. Detection Of Mutated