The study was aimed at investigation of the characteristic features of intestinal dysbacteriosis in man under extreme conditions and development of methods for correcting intestinal microflora for prevention of dysbacteriosis under such conditions. Microbiocenosis of the intestine was investigated in 99 practically healthy persons under model earth conditions and in 34 cosmonauts after space flights of various duration. The following factors influencing intestinal microflora in man under extreme conditions were studied: neuroemotional tension, hypokinesia, increased physical loads, isolation under conditions of altered gaseous environment and microclimate. The study revealed that the above factors participated in development of dysbacteriosis. It was shown that the neuroemotional stress played the main role in development of dysbacteriosis in man under extreme conditions: intestinal microflora responded under extreme conditions by decreased counts of bifidobacteria and lactobacilli participating in maintenance of the intact ecological barrier and colonization resistance. For preventing dysbacteriotic reactions of intestinal microflora it was recommended to use bifidobacterin tablets. Bifidobacterin correction was shown to be efficient in prevention of dysbacteriosis in persons under extreme conditions.