We show that for every computable tree T \mathcal T with no dead ends and all paths computable, and every D > T ∅ D >_T \emptyset , there is a D D -computable listing of the isolated paths of T \mathcal T . It follows that for every complete decidable theory T T such that all the types of T T are computable and every D > T ∅ D >_T \emptyset , there is a D D -decidable prime model of T T . This result extends a theorem of Csima and yields a stronger version of the theorem, due independently to Slaman and Wehner, that there is a structure with presentations of every nonzero degree but no computable presentation.