Relatively few studies have been conducted on the effect of diet on the intestinal microbiota. We followed the response of the fecal microbiota to a carbohydrate-restricted diet in an obese volunteer. The community composition was estimated through 16S rRNA gene sequence analysis. Bacteria of the Prevotella genus dominated the microbiota. Elimination of carbohydrates from diet triggered an immediate decrease in Prevotella, compensated by the increase in all other bacteria, and higher fluctuations in the community composition. Fecal microbiota changed back to its previous structure no later than 4 weeks after reintroducing carbohydrates in diet. Our results show the dependence of Prevotella on dietary carbohydrates and confirm the resilience of the microbiota to short-term dietary interventions. The potential link between Prevotella and obesity warrants further research.