Tritrichomonas foetus and Trichomonas vaginalis, parasitic protists of the urogenital tract, display a trophozoite and a pseudocyst stage. The ultrastructure of the trophozoite was compared with the pseudocyst form. The latter appears under unfavorable environmental conditions when the flagella are internalized, and a true cell wall is not formed. Although some authors consider this form as a degenerate stage, the cell behaves as a resistant form. Pseudocysts were found in natural culture conditions and also under induction by hydroxyurea or cycles of cooling and warming cultures. They were studied by light and scanning and transmission electron microscopy, using immunofluorescence and videomicroscopy. This report presents evidence that the trichomonad pseudocysts appear under stress conditions and that they are competent to divide. Pseudocysts differ from trophozoites in that: (1) the flagella are located in endocytic vacuoles and remain beating; (2) the axostyle and the costa are not depolymerized but present a curved shape; (3) the axostyle does not exhibit staining with antitubulin antibodies when the mitotic spindle is observed; (4) the mitotic process occurs within pseudocysts but differs from that described for trophozoites; (5) a nuclear canal is formed connecting the two spindle poles; and (6) the process is reversible if the cells are transferred to fresh medium.