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).