Microorganisms are selected in dental biofilm by the acidic environment created by sugar fermentation, but the effect of extracellular polysaccharide (EPS) on the counts of cariogenic bacteria is not clear. Dental biofilm was formed in situ for 13 days under exposure 8 times a day to distilled-deionized water, glucose + fructose or sucrose solutions. Mutans streptococci (MS) counts were not different among the groups, but lactobacilli (LB) were significantly higher in glucose + fructose and sucrose groups, without significant difference between them, irrespective of the higher insoluble EPS concentration in the sucrose biofilm matrix. The data suggest that exposure to sugar is more relevant for the predominance of LB in dental biofilm than for MS and that insoluble EPS does not change the counts of these microorganisms in the biofilm. Copyright (c) 2006 S. Karger AG, Basel406546549