Recent technical advances have improved the possibilities to classify anaerobic bacteria. The aim of the present study was to examine the validity of the time-honoured opinion that the faecal flora is dominated by bifidobacteria in breastfed infants but not in bottle fed ones. The composition of the faecal flora of 15 breastfed and of 7 bottle fed infants was followed from birth to 8 weeks. Strictly anaerobic conditions were carefully applied. At 5 days and 3 weeks the incidence of Staph. epidermidis was significantly greater in the breastfed group compared to the bottle fed one. We were unable to confirm earlier reports of difference in the anaerobic flora between breastfed and bottle fed infants. In both feeding groups Bacteroides dominated among the anaerobic bacteria and bifidobacteria occurred in less than half of the faecal specimens. The results of the study add to other recent observations that it has been increasingly difficult to demonstrate bifidobacteria in babies delivered in large, urban hospitals. We found different frequencies of bifidobacteria in infants from different wards, suggesting the importance of environmental factors in gut colonization after delivery.