The prevalence of the smooth and rough colonial morphotypes of Peptostreptococcus micros was examined with culture technique in 123 patients with adult periodontitis (age 24-68 years). Of all subgingival samples, 91% contained the smooth morphotype of P. micros. The smooth morphotype constituted a mean percentage of the total anaerobic viable biota of 6.0%, with a range of 0.02-35.7%. Of these samples, 49% contained colonies of the rough morphotype as well, with a mean percentage of the total anaerobic viable biota of 2.3% (range 0.01-16.2%). None of the samples contained only the rough morphotype. The total percentage of P. micros varied from 0.02-35.71% with a mean of 7.2%. No correlation was found between the prevalence of both morphotypes of P. micros and the age of the subjects or with loss of attachment or pocket depth