Bifidobacteria are a major microbial component of infant gut microbiota, which is believed to promote health benefits for the host and stimulate maturation of the immune system. Despite their perceived importance, very little is known about the natural development of and possible correlations between bifidobacteria in human populations. To address this knowledge gap, we analyzed stool samples from a randomly selected healthy cohort of 87 infants and their mothers with >90% of vaginal delivery and nearly 100% breast-feeding at 4 months. Fecal material was sampled during pregnancy, at 3 and 10 days, at 4 months, and at 1 and 2 years after birth. Stool samples were predicted to be rich in the species Bifidobacterium adolescentis, B. bifidum, B. dentium, B. breve, and B. longum. Due to high variation, we did not identify a clear age-related structure at the individual level. Within the population as a whole, however, there were clear age-related successions. Negative correlations between the B. longum group and B. adolescentis were detected in adults and in 1- and 2-year-old children, whereas negative correlations between B. longum and B. breve were characteristic for newborns and 4-month-old infants. The highly structured age-related development of and correlation networks between bifidobacterial species during thefirst 2 years of life mirrors their different or competing nutritional requirements, which in turn may be associated with specific biological functions in the development of healthy gut. M other-to-child transmission and temporal development of the human gut microbiota are population-based processes. Understanding these processes is essential to the identification of gutmicrobiota-associatedfunctionalities.Certainmembersofthe genusBifidobacteriumrepresentveryabundantearlycolonizersof the infant gut (1), making them a prime target for investigation. The high abundance of Bifidobacterium species in infants is considered to promote development and maturation of the immune system to sustain health (2‐4). Furthermore, our recent studies suggest that the succession of bifidobacteria is important for the proper immunological development (5, 6). Eight Bifidobacterium species have been associated with the human gastrointestinal tract (GIT): Bifidobacterium adolescentis, B. breve, B. longum subsp. longum, B. longum subsp. infantis, B. pseudolongum,B.bifidum,B.pseudocatenulanum,andB.dentium. Some bifidobacterial strains, e.g., B. pseudolongum, appear to be exclusively associated with adult gut microbiota, and some, especially B. longum subsp. infantis, are typically isolated from infants (7). The population-wise, age-related development of bifidobacteria in infants remains to be investigated. Due to the fact that there is evidence for previously uncharacterized diversity of bifidobacteria in the human gut (8, 9), we believe that to obtain a comprehensive description of bifidobacterial composition, the study should include a large number of individuals, and various techniques should be applied. It is also veryimportanttoavoidtargetingspecificbifidobacteria,sincethis may lead to the exclusion of as-yet-undiscovered, but potentially important bifidobacterial groups. The aim of the present study, therefore, was to describe bifidobacterial composition, temporal development, and possible correlationsinalarge,randomlyselectedcohortofmothersandtheir children using a combination of both culture-dependent and -independent techniques. This was done by analyzing the series of stool samples from the IMPACT cohort (10). The samples were collected during early and late pregnancy stages and at 3 days, 10 days, 4 months, and 1 and 2 years after birth. We present results showing a highly structured, age-related succession of bifidobacterial species within the study population, as well as correlations between the abundances of these species during thefirst 2 years of life.