Two strains of Atlantic salmon yolk sack fry (aquaculture and wild) were raised under germ-free conditions and, one week after hatching, exposed to water with either r- og K-selected microbial communities. The salmon fry was reared in the dark at 7 °C in tissue culture flasks, and 60 % of the volume of the water was exchanged three times a week, either with sterile synthetic freshwater (before exposure to bacteria) or with r- or K-selected water. Gut and water samples for microbial community analysis were collected two weeks after exposure to bacteria. Gut samples were collected by dissection and singly frozen in cryotubes. The water added to the fish flasks were collected every day of water exchange, and samples of the rearing water were collected at the end of the experiment. All water samples were collected with Sterivex filters and stored in the freezer. DNA was isolated, and an amplicon library of the V3+V4 region of the 16S rRNA gene was sequenced on an Illumina MiSeq run.