This study investigates the impact of dietary changes on the human colonic microbiota, focusing on the influence of resistant starch (RS) and non-starch polysaccharides (NSPs). Fourteen overweight male volunteers were provided with control, RS, NSP, and reduced carbohydrate weight loss (WL) diets over 10 weeks. Analysis of 16S rRNA sequences from six volunteers revealed 320 phylotypes, with 26 accounting for >1% of sequences. Samples clustered more by individual than by diet, but specific bacterial groups showed rapid changes in response to dietary shifts. Ruminococcus bromii relatives increased on the RS diet, while the uncultured Oscillibacter group increased on both the RS and WL diets. Eubacterium rectale relatives increased on the RS diet but decreased on the WL diet. Inter-individual variation was marked, with some volunteers showing significant changes in specific bacterial groups. The study highlights the dynamic nature of the colonic microbiota and the importance of individual differences in response to dietary changes.This study investigates the impact of dietary changes on the human colonic microbiota, focusing on the influence of resistant starch (RS) and non-starch polysaccharides (NSPs). Fourteen overweight male volunteers were provided with control, RS, NSP, and reduced carbohydrate weight loss (WL) diets over 10 weeks. Analysis of 16S rRNA sequences from six volunteers revealed 320 phylotypes, with 26 accounting for >1% of sequences. Samples clustered more by individual than by diet, but specific bacterial groups showed rapid changes in response to dietary shifts. Ruminococcus bromii relatives increased on the RS diet, while the uncultured Oscillibacter group increased on both the RS and WL diets. Eubacterium rectale relatives increased on the RS diet but decreased on the WL diet. Inter-individual variation was marked, with some volunteers showing significant changes in specific bacterial groups. The study highlights the dynamic nature of the colonic microbiota and the importance of individual differences in response to dietary changes.