The generation time of diplococcic viridans group streptococci, S. mitis is unusually prolonged when grown in rich broth containing xylitol (2% or higher concentrations). These xylitol-exposed streptococci predominantly grow in chains of varying lengths with heterogeneity of color: pink, purple and pink and purple combined. The pink portion of these bacterial chains appears to be thinner than the purple portion indicating a reduction in cell wall thickness. In support of optical microscopy, scanning electron microscopy of the same bacterial population demonstrates a considerable reduction of sizes but still in chains. In these chains, there is a variation in size indicating that all members of the diplococcic S. mitis population are not equally affected. Apparently, the partially affected viable population of S. mitis, becomes sensitive to colistin and nalidixic acid when grown in xylitol.