Recently, an "abnormal" faecal flora has been associated with Crohn's disease, a disease of unknown origin but with pertinent genetic predisposition. Therefore, it was investigated whether the faecal flora is under genetic influence in groups of monozygotic and dizygotic twins. Faecal floras of monozygotic twin siblings were found to be much more alike than those of dizygotic twin siblings. It is concluded that the resident faecal flora is under genetic influence and may be useful parameter to study possible genetic backgrounds of other diseases.