It is commonly assumed that antibiotics with a broad antibacterial spectrum affect the normal flora more than those with narrow spectrum. As exemplified in this overview, this in many cases is a misconception. Comparing two well-absorbed oral antibiotics like amoxycillin and cephradine, the former has more negative ecological consequences on the faecal flora. That is probably due to the effect of amoxicillin on enterococci, allowing resistant Escherichia coli to establish themselves in the ecological niche left vacant when the susceptible E. coli are eradicated. Absorption following oral administration may be of importance for the ecological pressure. However, norfloxacin which is absorbed incompletely, does not negatively affect the faecal flora. Other antibiotics may cause serious disturbances of the intestinal flora if excreted via the bile. Profound effects on the intestinal flora together with high incidences of diarrhoea have been demonstrated for ceftriaxone and cefoperazone while imipenem which has a broader spectrum but is not excreted via the bile, does not affect the faecal flora. It is concluded that factors other than the width of the antibacterial spectrum may be of importance for the ecological consequences of antibiotic treatment.