Anti-centrin monoclonal antibodies 20H5 and 11B2 produced against Clamydomononas centrin decorated the group of basal bodies as well as very closely attached structures in all trichomonads studied and in the devescovinids Foaina and Devescovina. Moreover, these antibodies decorated the undulating membrane in Trichomonas vaginalis, Trichomitus batrachorum, and Tritrichomonas foetus, and the cresta in Foaina. Centrin was not demonstrated in the dividing spindle and paradesmosis. Immunogold labeling, both in pre- and post-embedding, confirmed that centrin is associated with the basal body cylinder and is a component of the nine anchoring arms between the terminal plate of flagellar bases and the plasma-membrane. Centrin is also associated with the hook-shaped fibers attached to basal bodies (F1, F3), the X-fiber, and along sigmoid fibers (F2) at the pelta-axostyle junction, which is the microtubule organizing center for pelta-axostyle microtubules. There was no labeling on the striated costa and parabasal fibers nor on microtubular pelta-axostyle, but the fibrous structure inside the undulating membrane was labeled in T. vaginalis. Two proteins of 22-20 kDa corresponding to the centrin molecular mass were recognized by immunoblotting using these antibodies in the three trichomonad species examined. By screening a T. vaginalis cDNA library with 20H5 antibody, two genes encoding identical protein sequences were found. The sequence comprises the 4 typical EF-hand Ca++-binding domains present in every known centrin. Trichomonad centrin is closer to the green algal cluster (70% identity) than to the yeast Cdc31 cluster (55% identity) or the Alveolata cluster (46% identity).