ABSTRACT.    Reactive oxygen species are physiologically synthesized by every cell and oxidize a vast array of cellular constituents. If the oxidative damages are not repaired by anti-oxidative enzymes, oxidized macromolecules are no longer active and start accumulating within cells, thus affecting the organism lifespan. Elective targets of protein oxidation are methionine residues, in particular those which are carried exposed on the molecular surface. The modification of these residues to hydrophilic sulfoxides may result into changes that damage protein activity and stability. We investigated the molecular basis that Euplotes raikovi utilizes to repair these oxidative damages. At least three distinct genes encoding the enzyme methionine-sulfoxide reductase (Msr) were identified, and the complete structure of two of them (designated Er-msrB and Er-msrBA) was determined. Er-msrB encodes one Msr of type B responsible for the reduction of the oxidized methionine-R form. Er-msrBA contains two open reading frames, one of which is specific for a Msr of type B, while the second frame is specific for a Msr of type A responsible for the reduction of the oxidized methionine-S form. Er-msrB and Er-msrBA are expressed to different extents, and their expression appears to be an inducible phenomenon.