In this paper we present Scalable Position-Based Multicast (SPBM), a multicast routing protocol for ad-hoc networks. SPBM uses the geographic position of nodes to provide a highly scalable group membership scheme and to forward data packets with a very low overhead. SPBM bases its multicast forwarding decision on whether there are group members located in a given direction or not, allowing for a hierarchical aggregation of group members contained in geographic regions: the larger the distance between a region containing group members and an intermediate node, the larger can this region be without having a significant impact on the accuracy of the direction from the intermediate node to that region. Because of aggregation, the overhead for group membership management is bounded by a small constant while it is independent of the number of multicast senders for a given multicast group. We investigate the performance of SPBM by means of simulation, including a comparison with ODMRP.