Certain nitrothiazole derivatives, such as niridazole (Ba 32644, Ambilhar), exert pronounced activities against several trichomonad strains when tested under anaerobic conditions comparable to those of the nitroimidazole derivatives. Few compounds of the nitrothiazole group, however, are markedly less active in vitro. The in vivo activity of the nitrothiazole derivatives as determined in mice infected subcutaneously with T. foetus, is as good as that of metronidazole. A nitrothiazole derivative with a laminar side chain is inactive in vivo after oral administration, although it shows pronounced antitrichomonad activity in vitro.