The effects of the sucrose derivatives, lylose (palatinose) and palatinit, and of the artificial sweetners saccarin and aspartame, on the growth, acid production, and glucan synthesis by oral streptococci were assessed. The four sugar substitutes were not fermented by any of the test bacteria. Saccharin inhibited growth and acid production by strains of S. mutans, S. sanguis, and S. salivarius grown in the presence of either sucrose or glucose. Glucan synthesis by S. mutans glucosyltransferase was inhibited by saccharin, palatinose and palatinit; inhibition was non-competitive with sucrose. The uptake of 14C-xylitol and the effect of xylitol on the uptake of 14C-glucose by S. mutans, S. sanguis, S. salivarius and S. mitis were studied in resting cell cultures. Xylitol did not significantly effect the rapid uptake of l4C-glucose by the oral streptococci. However, l4C-xylitol was taken up at a slow rate by several strains of bacteria. The major metabolite of l4C-xylitol extracted from these cells was tenatively identified as xylitol-5-phosphate.