The subgingival plaque around both teeth and implants was analysed by means of differential phase-contrast microscopy. It was noted that, in comparison to natural teeth, the subgingival samples from implants were more frequently too small to provide adequate bacterial counts. In 24 partially edentulous patients (with implants and teeth in the same jaw), no significant differences in the distribution of bacterial morphotypes could be found between implants and natural teeth. The %s of coccoid cells, motile rods, spirochetes and other bacteria were 65.8, 2.3, 2.1, and 29.8 for implants and 55.6, 4.9, 3.6, and 34.9 for teeth, respectively. However, when the plaque composition on the implants of fully edentulous patients was compared with those of teeth or implants of partially edentulous patients (with teeth and implants in the same and/or opposite jaw), significant differences appeared. In fully edentulous patients, more coccoid cells (71.3%) and significant fewer motile rods (0.4%) and spirochetes (0.0) were found around the implants. The results suggest that teeth may serve as a reservoir for the bacterial colonisation of titanium implants in the same mouth.