This paper studies two-hop cooperative demodulate-and-forward relaying using multiple relays in wireless networks. A threshold based relay selection scheme is considered, in which the reliable relays are determined by comparing source-relay SNR to a threshold, and one of the reliable relays is selected by the destination based on relay-destination SNR. The exact bit error rate of this scheme is derived, and a simple threshold function is proposed. It is shown that the network achieves full diversity order (N + 1) under the proposed threshold, where N is the number of relays in the network. Unlike some other full diversity achieving protocols in the literature, the requirement that the instantaneous/ average SNRs of the source-relay links be known at the destination is eliminated using the appropriate SNR threshold.