This study examined the effects of maternal and/or post-weaningBacillus altitudinissupplementation on the microbiota in sow colostrum and faeces, and offspring digesta and faeces. Sows (n= 12/group) were assigned to: (1) standard diet (CON), or (2) CON supplemented with probioticB. altitudinisspores (PRO) from day (d)100 of gestation to weaning (d26 of lactation). At weaning, offspring were assigned to CON or PRO for 28d, resulting in: (1) CON/CON, (2) CON/PRO, (3) PRO/CON, and (4) PRO/PRO, after which all received CON. Samples were collected from sows and selected offspring (n= 10/group) for 16S rRNA gene sequencing.Rothiawas more abundant in PRO sow colostrum. Sow faeces were not impacted but differences were identified in offspring faeces and digesta. Most were in the ileal digesta between PRO/CON and CON/CON on d8 post-weaning; i.e.Bacteroidota,Alloprevotella,Prevotella,Prevotellaceae,Turicibacter,CatenibacteriumandBlautiawere more abundant in PRO/CON, withFirmicutesandBlautiamore abundant in PRO/PRO compared with CON/CON.Lactobacilluswas more abundant in PRO/CON faeces on d118 post-weaning. This increased abundance of polysaccharide-fermenters (Prevotella,Alloprevotella,Prevotellaceae), butyrate-producers (Blautia) andLactobacilluslikely contributed to previously reported improvements in growth performance. Overall, maternal, rather than post-weaning, probiotic supplementation had the greatest impact on intestinal microbiota.