Let X be a Banach space, A : D(A) ⊂ X → X the generator of a compact C0-semigroup S(t) : X → X, t ≥ 0, D a locally closed subset in X, and f : (a, b)×X → X a function of Caratheodory type. The main result of this paper is that a necessary and sufficient condition in order to make D a viable domain of the semilinear differential equation of retarded typeX → X, t ≥ 0, D a nonempty subset in X. Let q and T be positive numbers and −∞ ≤ a < b ≤ +∞. Given t 0 ∈ (a, b), a function x : [t 0 − q, t 0 + T ] → X and t ∈ [t 0 , t 0 + T ], define x t : [−q, 0] → X by x t (θ) = x(t + θ) for all θ ∈ [−q, 0]. In this paper, we discuss the semilinear differential equation of retarded type,2) where C([−q, 0]; X) denotes the Banach space of continuous X-valued functions on [−q, 0] with supermum norm, f : (a, b) × X → X and t 0 ∈ (a, b).We say that D is a viable domain for (1.1) if for each t 0 ∈ (a, b), φ ∈ C([−q, 0]; D) , there exists at least one mild solution u :We recall that by mild solution to (1.1) and (1.2) we mean a continuous function u : [t 0 − q, t 0 + T ] → X, satisfying u(t 0 + θ) = φ(θ)