The cause of IBS has yet to be established, however the role of the gut microbiota in modulating symptoms is the subject of significant recent research. This randomised, double-blind, placebo controlled trial of 88 healthy subjects with chronic diarrhoea, aimed to assess the impact of ingestion of probiotic strain Bacillus subtilis C-3102 over an 8 week period on patient symptoms. Approximately 8.5 billion spores of Bacillus subtilis C-3102 were administered daily to the treatment group, once a day, immediately after breakfast. The results, after 8 weeks of ingestion, showed: an improved Bristol Stool Chart score; a reduction in stool frequency; significantly decreased abdominal sounds; and significantly altered relative abundance of Lachnospira, Actinomyces and SMB53. Intake of 4 weeks or more were required to achieve these results. Even though this was a small study, nutrition and lifestyle practitioners looking to support the digestive health of IBS patients may wish to consider adding this probiotic strain to their recommendations on the grounds of improved symptoms of diarrhoea.