or differentiation between faecal coliforms and members of the aerogenes group on the basis of citrate utilization. It is recommended by BIS committee under specification IS:5887 (Part I) 1976, reaffirmed 2005. SIMMONS CITRATE AGAR (as per BIS) is used for differentiation between faecal coliforms and members of the aerogenes group on the basis of citrate utilization. Initially the citrate medium was developed by Koser containing ammonium salt as the only nitrogen source and citrate as the only carbonsource for differentiating Escherichia coli and Enterobacter aerogenes by IMViC tests. Later on Simmons modified Kosers formulation by adding agar and bromothymol blue. Sodium citrate and ammonium dihydrogen phosphate serves as the sole nitrogen and carbon source. Bromo thymol blue is the pH indicator. The organisms produce an alkaline reaction which is indicated by the change in colour from green to blue.