Flow cytometric analysis of the binucleated protozoan parasite Giardia intestinalis gave DNA histograms with a broad Gl peak and a definable G2 + M peak with twice the DNA content of Gl. Twenty-four hour treatment with metronidazole arrested cell cycle progression of susceptible trophozoites in the G2 + M phase, but had no effect, even at toxic doses, on the DNA histogram of a line selected for resistance to metronidazole. Furazolidone was inhibitory to both stocks, causing an arrest in the S and G2 + M phases. Inhibitors of the mammalian cell cycle were also tested. Hydroxyurea, which blocks mammalian cells in Gl/S, and razoxane, which blocks in G2 + M, arrested trophozoites in the G2 + M phase whereas colchicine and gamma-irradiation had little or no effect on the cell cycle of G. intestinalis. These results suggest that the cell cycle of G. intestinalis may be controlled in a different manner from mammalian cells.