The aims of the study were to determine the genetic diversity of the rare Actinobacillus actinomycetemcomitans serotype d and to compare the ability of the repetitive extragenic palindromic element (REP)-based polymerase chain reaction (PCR) with that of the arbitrarily primed (AP)-PCR to discriminate between and within A. actinomycetemcomitans serotypes. The material included 26 A. actinomycetemcomitans serotype d isolates, 3 reference strains, and 21 A. actinomycetemcomitans isolates, representing the previously described 17 AP-PCR genotypes from 4 serotypes (a, b, c and e). Among A. actinomycetemcomitans serotype d isolates (n=26), the AP-PCR primer distinguished 2 genotypes, whereas the REP-primer pair (REP1R-I and REP2-I) and the (GACA)(4) primer each produced one genotype. Among the total of 50 A. actinomycetemcomitans isolates, REP-primer pair distinguished 6 genotypes, the primer (GACA)(4) 7 genotypes, and the AP-PCR 19 genotypes. Among A. actinomycetemcomitans serotype a isolates (n=6), REP-primer pair yielded 3 genotypes and (GACA)(4) and AP-PCR primer 4 genotypes, and among serotype e isolates (n=6) 3 genotypes. All serotype b isolates (n=7), representing the AP-PCR genotypes 2, 9, 8, 12, 13, 16 and serotype c isolates (n=5), AP-PCR genotypes 3, 4, 14, 15, belonged to the (REP1R-I and REP2-I)-PCR genotype 4 and to the (GACA)(4)-PCR genotype 4. In conclusion, based on both the AP-PCR method and the less discriminative REP-PCR methods, the present genotyping results indicated limited genetic diversity among serotype d isolates of A. actinomycetemcomitans.