Aims The intestinal microbiota of newborn calves is largely unexplored even if it is of great significance for their future health. Therefore, the aim of the study was to gain a better insight into the development dynamics of certain bacterial groups during the first week of life. Methods and Results Faecal samples of healthy Simmental calves (dual-purpose breed; n = 80), bottle fed and raised in a dairy farm were taken immediately after birth and at 6/12/24/48/72/168 h (h) after birth. Samples were analysed using cultural, biochemical and molecular-biological methods. The aerobe, anaerobe, Enterobacteriaceae and Enterococcus counts of healthy calves increased significantly between 6 and 24 h postpartum (P <0 center dot 05). Apart from the anaerobes, bacterial counts decreased after reaching a plateau at 24-48 h. Enterococcus faecalis was detected in significantly higher counts compared to E. faecium (P <0 center dot 05). Lactobacilli developed more slowly and increased until day 7 after birth to a mean value of 6 center dot 8 x 10(7) CFU per g. MALDI-TOF analysis of 2338 lactobacilli isolates resulted in 36 different species. Conclusions Lactobacillus reuteri became the most common Lactobacillus sp. during the first week of life. Significance and Impact of the Study This fact seems to be very important for the calf's intestinal health because L. reuteri is known to show in vitro bactericidal effects against bacterial pathogens and anti-infective activities against rotaviruses and Cryptosporidium parvum.