In the paper we generalize a fibre criterion for a polynomial f to belong to a primary ideal I in the polynomial ring K[X, Y ]. We also investigate the general case where the ideal I is not primary. Let {X1, . . . , Xn} be any set of variables. We shall write K[X] instead of K[X1, . . . , Xn]. If f ∈ K[X,Y ], where X and Y are sets of variables, K is an algebraically closed field, Y = {Y1, . . ...