The present study has been performed to see the intrafamilial transmission of periodontopathic organism Actinobacillus actinomycetemcomitans in Koreans having various froms of periodontal diseases. 17 clinical isolates from 8 periodontal patients and 20m clinical isolates from their 8 family members were grown anaerobically for the serotyping and the extraction of genomic DNA. The DNA was digested with restriction endonucleases (EcoRI+HindIII) and plasmid pAA 2097(kindly provided by Dr. DiRienzo, Univ. of Pennsylvania) including 4.7kb-size randlomly clone probe for restriction length pleomorphism analysis(RFLP). RFLP patterns of reference serotypes a, b, c, d, and e were used as the genotypes A, B, C, D, and E, respectively for comparison of genotypes of clinical isolates. 28 out of total 37 clinical isoltes belonged to either one of 5 basic gentotypes and 9 remaining isolats did not fall into any types, and hence were designate as non-type(NT). Genotype C were the most frequently found one(35.1%) and genotype B has not isolated. Intrafamilial transmission of bacteria between spouses, brothers and sisters, and parents and their offsprings, resepctively could well be demonstrated by comparing RFLP patterns. There were not any specific genotypes which showed predominance over others in terms of transmission.