Abstract. Suppose α is an orientation-preserving diffeomorphism (shift) of R + = (0, ∞) onto itself with the only fixed points 0 and ∞. In [6] we found sufficient conditions for the Fredholmness of the singular integral operator with shiftacting on L p (R + ) with 1 < p < ∞, where P ± = (I ± S)/2, S is the Cauchy singular integral operator, and W α f = f • α is the shift operator, under the assumptions that the coefficients a, b, c, d and the derivative α ′ of the shift are bounded and continuous on R + and may admit discontinuities of slowly oscillating type at 0 and ∞. Now we prove that those conditions are also necessary.