Thirteen samples of ancient human dental calculus were analyzed by scanning electron microscopy (SEM). Samples came from ten adults from the necropolis Znojmo-Hradiště which is dated to the Great Moravian period (the end of the 9th and beginning of the 10th century AD). SEM allowed observation and measurement of the excavated calculus objects with submicrometer resolution. Therefore it was possible to estimate plant/vegetable fibers and all bacterial morphological types like rods, cocci, spirals and filamentous forms. This confirms high oral bacterial diversity of medieval agriculturalists which is in agreement with recent molecular studies, but without destruction of samples and with lower costs. Presence of plant/vegetable fibers in dental calculus validated the vegetable part of the diet of early medieval Slavs found directly in excavated human skeletons.