We address the problem for finding the K best path trees connecting a source node with any other nonsource node in a directed network with arbitrary lengths. The main result in this paper is the proof that the kth shortest path tree is adjacent to at least one of the previous (k−1) shortest path trees. In this paper, we consider the K shortest path trees problem as the problem to determine the K best basis trees (solutions) of the classical mathematical formulation of the SP problem.The determination of the K shortest paths in a network has a wide range of applications. Some of them are cited in Eppstein [8]. We are unaware of any previous references to this problem in the literature. A proof is offered which shows that the kth best basis tree is adjacent to at least one of the previous k−1 best basis trees. In other words, the kth best solution is obtained from one of the previous best solutions by exchanging an arc in the basis tree for an arc outside of the basis tree. This results allows an algorithm to be designed running in