The effects of hand and ultrasonic instrumentation on root surface topography and subgingival plaque were studied. One approximal subgingival root surface on each of two single rooted teeth of six patients was root planed. The opposing proximal surfaces were not treated and served as controls. On each patient, the two test surfaces were randomly treated by hand or ultrasonic instrumentation (Cavitron) until clinically smooth. The teeth were extracted, divided into mesial and distal sides, and processed for scanning electron microscopy (SEM). The observations were restricted to an area 2 mm by 3 mm just coronal to the plaque-free zone. The root surfaces were scanned by SEM, both at low (x 100) and at higher magnifications (x 5000). At low power, a photomontage was produced of the surface area of the test teeth. At low magnifications (x 100, x 500), the topography resulting from hand instrumentation appeared smooth with parallel scratches compared to scaly but flat surfaces with occasional deep gouges with the ultrasonic method. At high magnifications (x 5000), tooth surfaces were similar in topography when instrumented by either hand or ultrasonic methods. Root surfaces were covered in some areas by a fibrillar-like membrane that resembled fibrin. Individual bacteria were found on the surfaces of this material. On other areas not covered by this membrane, bacterial plaque was only observed when tooth surfaces had not been contacted by either hand or ultrasonic instrumentation.