An assessment of microflora beneath restorations indicates how the antimicrobial activity of filling materials on residual microorganisms and whether the material prevents a penetration of microorganisms after its application. The aim of the present study was to determine a microbial spectrum under composite fillings compared to that under amalgam fillings. Ten composite and 5 amalgam fillings were evaluated for occlusal loss of material, marginal gaps, and secondary caries. After the removal of the fillings, a sample of carious dentine just below the filling was taken under sterile conditions and stored in a prereduced transport medium for anaerobs and immediately transfered to a laboratory for microbial diagnosis. Clinical examination showed in 50% of all restorations occlusal loss of material and marginal gaps. In 9 out of 10 composite fillings and 2 out of 5 amalgam fillings secondary caries was diagnosed. The ratios of aerobic to anaerobic flora were comparable. (under composite 11.4% : 88.6%) and amalgam 15.4% : 84.5%). The microbial variety under composite fillings was much greater compared to amalgam and it was similar to infected root canals (strict anaerobic gram-negative rods as Fusobacterum spp. Porphyromonas spp.). Quantitatively, there were up to 8 times more microorganisms under composite fillings. Beneath amalgam we found microbes similar to the flora of carious dentine and carious plaque, with anaerobe and facultative anaerobic gram-positive rods dominating. The kind of filling material may have an important influence on quantity and quality of the microbial spectrum in dental cavities. Associations between anaerobic gramnegative flora under composites and infected root canals need further investigations. Keywords: dental materials, microbiology, composite, amalgam