Glucosylsucrose and maltosylsucrose are principal constituents of coupling sugar, which is prepared from starch and sucrose and has a similar taste to cane sugar. Cell suspensions of Streptococcus mutans grown in a continuous culture under sucrose limitation produced only a small amount of acids from these sugars. The rate of hydrolysis of these sugars by cell-free extract of S. mutans was less than 10% of that of sucrose. Addition of sucrose or glucose lowered the pH in deposits of S. mutans but addition of glucosylsucrose or maltosylsucrose did not. We conclude from these findings that S. mutans ferments these sugars to a limited extent.