Guar gum is widely used in the food industry as a thickening agent. Guar and other galactomannans are ingested as a normal part of the human diet. Guar is completely degraded in the large intestine. Often large amounts of gas are produced. The objective of the study was to determine which species are responsible for the degradation of guar in the GI tract. It was observed that only a limited number of species is able to degrade and ferment guar. Guar degrading strains could be isolated from faecal samples of all volunteers and in 90% of the saliva of volunteers. The main species isolated from humans were Bifidobacterium dentium and Clostridium butyricum. From several samples of animal faeces Streptococcus bovis could be isolated. In addition some strains of Bacteroides ovatus were able to degrade guar to a limited extent. Fermentation resulted in the production of short-chain fatty acids and, when Cl. butyricum was present, in a large gas production. Competition experiments showed that Cl. butyricum degrades guar faster than both other species under simulated physiological conditions. It was concluded that Cl. butyricum is the main guar degrading species and the causative agent of the gas formation after guar intake.