The main result of this paper is a non-differentiability Theorem (Theorem 1) for a superposition operator, i.e. the Nemytsky operator. Two more differentiability theorems are proved (Theorems 2 and 3). The paper “Osservazioni sulla linearizzazione di un operatore differenziale” (following the present one in these “Rendiconti”) contains some consequences of Theorems 1, 2, 3 concerning certain problems connected with the study of a special class of nonlinear differential operators.