A concrete dynamic-data type is just a partial algebra with predicates such that for some of the sorts there is a special predicate defining a transition relation. An abstract dynamic-data type (ad-dt) is an isomorphism class of such algebras. To obtain specifications for ad-dt's, we propose a logic which combines many-sorted first-order logic with branching-time combinators. We consider both an initial and a loose semantics for our specifications and give sufficient conditions for the existence of the initial models. Then we discuss structured specifications and implementation.