In the article the survey of data on morphology and ultrastructure of flagellates of the order Trichomonadida is given with special attention paid to Monocercomonadidae and Trichomonadidae. The evolution within these flagellates is considered as a series of changes leading to formation of undulating membrane and reinforcement of the cytoskelet. In both families other two evolutionary tendencies may be also observed: reduction of the flagellar apparatus (in Monocercomonadidae) and enlargement of the number of free flagella (in both families).
Relations of particular genera of Monocercomonadidae and Trichomonadidae with groups of hosts are discussed. Main hosts of these flagellates are insects (mainly termits) and cold-blooded vertebrates (amphibians and reptiles); the proper location within the host body is the intestine. Some representatives of these flagellates have changed their host and location and occur now in warm-blooded vertebrates (birds and mammals) and in man, usually in organs other than the intestine. Change of localization within the host body is probably responsible for growing pathogenecity of these flagellates.