The fractional order differential equationis studied, where A is an operator generating a strongly continuous one-parameter semigroup on a Banach space X, D α t is the Riemann-Liouville fractional derivative of order α ∈ (0, 1), γ > 0 and f is an X-valued function. Equations of this type appear in the modeling of unidirectional viscoelastic flows. Well-posedness is proven, and a subordination identity is obtained relating the solution operator of the considered problem and the C 0 -semigroup, generated by the operator A. As an example, the Rayleigh-Stokes problem for a generalized second-grade fluid is considered.