The composition of subgingival plaque, from healthy and periodontally diseased regions, as well as the clinical periodontal condition of 6 patients, 17 to 24-years-old suffering from juvenile periodontitis were examined. 6 older patients with rapidly progressive periodontitis served as a control. Samples of subgingival plaque were taken from first molars and central incisors and were analysed morphologically by dark-field microscopy. In the control group in healthy regions the ratio between non-motile and motile bacteria was 27:1 and about 1:1 in deep pockets. In the juvenile periodontitis group in diseased regions, motile bacteria made up only 1/4 of the bacteria present. The results clearly show that our relatively old patients with juvenile periodontitis had a completely different microflora in their periodontally diseased regions than patients with common periodontitis. As regards our observations by dark-field microscopy, one can assume, however, that with increasing age, there might be a shift of the flora associated with juvenile periodontitis lesions from a rather simple composition to a more complex composition very similar to the flora seen in rapidly progressive adult periodontitis lesions.