Immunoblotting studies, using a polyclonal antibody specific for the alpha-tubulin of ciliates and an anti-beta-tubulin monoclonal antibody, demonstrated that the tubulins of the ciliates Euplotes and Oxytricha show alpha/beta inversion although less accentuated than that observed in Paramecium and Tetrahymena. Results suggest that (a) the alpha/beta inversion may be a common characteristic within the Phylum Ciliophora; and (b) the electrophoretic behaviour of the alpha-tubulin may be useful for establishing evolutionary relatedness between the ciliates.