Abstract: Objective　To investigate the aciduric bacteria in different stages of the fissure caries in orderto determine potential roles of the microflora in the development of dental caries.Methods　Plaque sampleswere taken from10 incipient carious fissures of the first permanentmolars. Plaque sampleswere also collected from sound fissures of the first permanentmolars in 10 other sub- jects who kept caries free over the past 2 years and from the above 10 subjects. The predominant bacteria were isolated by using both the most probable method in media at pH7·0 and 5·2, and the conventional plating methods.Streptococcus spp. was identi- fied by means of API 20 Strep commercial kit.Results　S.mutanswas the predominant aciduric bacteria in fissures of caries- free children and more frequently recovered at neutral pH. Whilst,S.sanguiswas predominant at pH5·2 in sound and carious fissures of caries-active children. The proportion of Gram-positive rods at pH5·2 was 82%, significantly greater than that of 61% at pH7·0.Conclusion　The ecology of plaque is complex and the composition ofmicroflora at each stage of caries could be differ- ent. The composition of bacteria was consistent with the changes of local pH in the plaque.

Key words: predominant aciduric microflora, fissure caries, plaque