The sequence file contains Illumina MiSeq 16S rRNA gene sequences from two cohorts. The samples from the first cohort were collected when the tumours had grown to 50 mm3, and consisted of 15 mice in five groups (n = 3 per group) fed diets comprising: low fibre (LF), low fibre with butyrate (LF+B), high mixed fibre (mixed HF), high insoluble fibre (insoluble HF), high soluble fibre (soluble HF). The samples from the second cohort were collected when the tumours had reached 350 mm3 either without (n = 3) or following (n = 8) radiation, and consisted of four groups fed diets comprising: low fibre, high soluble fibre, high insoluble fibre and high mixed fibre.