With human saliva as an inoculum, microcosm dental plaques were grown on dental amalgam in a constant-depth film fermenter (CDFF) in the presence (sue + ve) and absence (sue - ve) of sucrose. The biofilms were then exposed to 0.2% chlorhexidine gluconate (CHG) for 1, 5, or 60 min and the survivors enumerated. Suc + ve biofilms had higher proportions of streptococci but lower proportions of veillonellae than suc - ve biofilms. Exposure to CHG for 1 min reduced the viable count of suc - ve and suc + ve biofilms by 53% and 89% respectively, In both cases, reductions were mainly attributable to killing of streptococci and actinomyces. After 60 min of exposure, 4% of the bacteria in the suc - ve and 2% in the suc + ve biofilms remained viable. This study has shown that large numbers of bacteria in microcosm dental plaques can survive a 1-min exposure to 0.2% CHG and that even after a 60-min exposure, substantial numbers of bacteria remain viable.