Colonization by Bifidobacterium occurs generally within 4 days of life. The new method FISH has been applied for molecular detection of Bifidobacteria. The study was carried out on 26 healthy newborns delivered by vaginal delivery. Breast-fed infants harbor a gastrointestinal flora characterized by an increased concentration of Bifidobacterium cells (by a factor of 1.75). In artificial alimentation, some infants either did not harbor any Bifidobacterium or showed lower numbers of Bifidobacterium. Moreover, male newborns show higher numbers of Bifidobacterium, but in both sexes the predominance of Bifidobacterium is evident after maternal alimentation.