A synthetic basal salts medium containing inositol as the sole C and energy source and bromothymol blue as pH indicator, selectively supports the growth of K. pneumoniae and Serratia spp. On this medium incubated at 37.degree. C for 24 h, all cultures of K. pneumoniae, S. liquefaciens and S. marcescens from yellow colonies. The medium may be used selectively for the primary isolation of these bacteria from feces and as a differential medium for their isolation from [human] saliva and urine.