This research is a continuation of the series of studies of the parietal microbiota of the bowel tissue samplings by the gas chromatography and mass spectrometry (GCMS) method [G.A. Ossipov et al.//Journal of the Society of Russian Gastroenterologists, 2001, 1:54-69]. The purpose was to study a number of new microorganisms in view of new data on the composition of their fatty acids (FA) and aldehydes; to confirm the presence of a number of bacteria, fungi and aerobic actinomycetes revealed earlier by FA markers in the composition of the bowel parietal microbiota by their isolation in a pure culture; to amend the estimation of the clinical value of changes in the composition of the human bowel parietal microflora in case of irritable bowel syndrome (IBS) and antibiotics-associated diarrhea (AAD). We examined 31 patients with IBS with predominating diarrhea, 18 patients with AAD and 3 volunteers (a control group). We studied the blood samples, tissue samplings of the mucous coat of the jejunum, ileum and colon and composition of healthy people's feces. The GCMS method was applied. Morphology of defined strains was controlled by methods of light and scanning electron microscopy. We found a substantial portion of eubacteria among the bowel microorganisms and specific changes of their species in case of IBS and AAD. Taking into account their physiological and biochemical activity, when regulating their concentration one can expect at least the same effect as when regulating the number of bifidobacteria and lactobacilli in treatment of intestinal pathologies and other diseases related to bowel dysbacteriosis. The analysis of the feces microbiota using the GCMS method by FA of parietal microorganisms provides reliable data on their number both in feces and in tissue samplings. We found a substantial portion of eubacteria among other bowel microorganisms (27% in the jejunum and 16% in the colon) and specific changes of their species in case of IBS and AAD. The concentration of streptomycetes, rhodococci and other members of the Actinomycetales order becomes dozens times more and/or reduces in pathological states. The tenfold concentration of markers of lactobacilli and bifidobacteria in some diseases stimulates the differentiated application of widespread probiotics based on these bacteria.