In this study we investigated the effect of membrane filtration in RAS on water microbiomes and the hindgut microbiomes of the reared Atlantic salmon parr. The experiment is described in Fossmark et al. 2020, Aquaculture https://doi.org/10.1016/j.aquaculture.2020.735268. Samples from water and guts were taken at two time points in the experiment from the RAS with membrane filtration (mRAS) and a control RAS without membrane filtration (cRAS). Microbial communities were characterized by Illumina sequencing of amplicons representing the v3+v4 regions of the 16S rRNA gene and based DNA extracts.