Protistan cells employ a wide variety of strategies to reinforce and give pattern to their outermost cortical layers. The hypotrich ciliate Euplotes possesses a continuous monolayer of scales or plates, located within flattened membranous sacs (`alveoli") just below the plasma membrane, and this provides rigidity and form to the cell. Using immunological techniques, the major proteins comprising these `alveolar plates" have been identified and termed α-, β-, and γ -plateins. The present report describes work leading to the molecular characterization of three plateins, α1 and α2 (predicted Mrs of 61 and 56 kDa) and a β/γ form (Mr=73 kDa). All three proteins have features that are hallmarks of articulins, a class of cytoskeletal proteins that has been identified in the cortex of a wide variety of protistan cells, including certain flagellates, ciliates, dinoflagellates and Plasmodium. Chief among these common features are a prominent primary domain of tandem 12-amino acid repeats, rich in valine and proline, and a secondary domain of fewer, shorter repeating units. However, variations in amino acid use within both primary and secondary repetitive domains, and a much more acidic character (predicted pIs of 4. 42555. 9), indicate that the plateins represent the first proteins in a new subclass or family of articulins.