Fecal samples of two newborn calves were analyzed to detect bacterial succession in their gut. Escherichia coli and Bifidobacterium animalis were the initial colonizers, followed by species closely related to the genera Bacteroides, Clostridium and Faecalibacterium. Change of diet was connected with shifts of bacterial population and with the occurrence of many bacterial species that have not been cultured up to now. The diet change corresponded with an alteration in a volatile-fatty-acid concentrationin fecal samples.