In this paper, a new class of distributed space-time block codes (DSTBCs) is introduced. These DSTBCs are designed for wireless networks which have a large set of nodes N but at any given time only a small, a priori unknown subset of nodes S /spl sub/ N can be active. In the proposed scheme, the signal transmitted by an active node is the product of an information-carrying code matrix and a unique node signature vector. It is shown that existing STBCs designed for N/sub c/ /spl ges/ 2 co-located antennas are favorable choices for the code matrix guaranteeing a diversity order of d = min{N/sub S/, N/sub c/} if n/sub S/ nodes are active. For the most interesting case N/sub S/ /spl ges/ N/sub c/ the performance loss entailed by the distributed implementation is analytically characterized. Furthermore, efficient methods for the optimization of the set of signature vectors are provided. Possible applications of the proposed DSTBCs include ad hoc and sensor networks employing decode-and-forward relaying.