Abstract Conjugation in the free-living ciliate Tetrahymena thermophila is an inducible developmental system which results in a synchronized reorganization of the genetic material in both mates of a pair. The cytological events were followed by Feulgen stainings of simultaneously mating cells and protein synthesis was revealed using [ 35S]methionine pulse labelling and two-dimensional gel electrophoresis. At least 33 proteins, including 24 conjugation-specific proteins, with apparent molecular weights ( M r ) between 61 and 200 × 10 3 are stimulated during conjugation. Two slightly acidic proteins ( M r 89 and 73 × 10 3, respectively) are stimulated shortly after mixing of mating-competent cells and mainly before tight pairs are formed. Ten proteins are stimulated during meiosis, and two of these ( M r 90 and 78 × 10 3, respectively) are particularly interesting, since they are highly stimulated and more basic (pI values around 8.5) than most other proteins detected. Twelve proteins are stimulated essentially between pairing and early macronuclear development, three are stimulated from shortly before zygote formation and during the postzygotic divisions, and six are stimulated during late conjugation, at various parts of macronuclear development. The functions of the conjugation-stimulated proteins are discussed.