An authenticated group key exchange (AGKE) scheme allows a group of users in a public network to share a session key which may later be used to achieve desirable cryptographic goals. In the paper, we study AGKE schemes for dynamically changing groups in ad hoc networks, i.e., for environments such that a member of a group may join and/or leave at any given time and a group key is exchanged without the help of any central sever. Difficulties in group key managements under such environments are caused by dynamically changing group and existence of no trustee. In most AGKE schemes proposed so far in the literature, the number of rounds is linear with respect to the number of group members. Such schemes are neither scalable nor practical since the number of group members may be quite large and the efficiency of the schemes is severely degraded with only one member's delay. We propose an efficient provably secure AGKE scheme with constant-round. The propose scheme is still contributory and efficient, where each user executes three modular exponentiations and at most O(n) XOR operations.