The treatment of tooth surfaces with chlorhexidine varnish may lead to long-lasting suppression of mutans streptococci in dental plaque. Microbiological observations following varnish treatment suggest that this prolonged suppression might be caused by bacterial interference. To investigate whether physiologically related organisms, such as other Streptococcus species, compete with mutans streptococci in the ecosystem, we have analyzed streptococcal populations on the tooth surface before and after chlorhexidine varnish treatment. Occlusal surfaces with high numbers of mutans streptococci were selected in human volunteers and treated with chlorhexidine varnish. Analyses of sequentially collected plaque samples confirmed that S. oralis-group streptococci returned to baseline levels shortly after the chlorhexidine application, while Actinomyces naeslundii populations reached prestudy or even higher levels only several days after treatment. Mutans streptococci, however, were below the detection level in the 14-day samples, except in 1 individual. The pattern of recolonization by individual Streptococcus species after chlorhexidine application closely resembled that of cleaned enamel surfaces: S. oralis and S. sanguis were primary colonizers while S. gordonii became dominant at a later stage. It is concluded that after intensive chlorhexidine treatment, a normal oral microflora reestablished, characterized by low proportions of mutans streptococci.