Summary Germ nuclei of Blepharisma japonicum begin meiosis within a few hours when cells of complementary mating types conjugate. We synchronized the onset of conjugation and treated cells in different stages of meiosis with 10 μg/ml cycloheximide which strongly inhibits protein synthesis in this ciliate. Cycloheximide arrested meiosis at six stages: I, between pairing of cells and swelling of germ nuclei; II, leptotene; III, zygotene; IV, pachytene; XI, interkinesis; XII, prometaphase II. Five of these arrests were rersible. Puromycin (250–500 μg/ml) also inhibited the progress of meiosis, though to lesser extents. We propose that the progession of meiosis of B. japonicum requires at least six proteins which are synthesized sequentially during meiosis.