Existing approaches to service discovery have been developed primarily for environments with a fixed network backbone and typically rely on centralized components being accessible to potential service clients at any given time. The characteristic lack of a designated service infrastructure in combination with the highly dynamic nature of the underlying network topology renders such discovery mechanisms unsuitable for mobile ad hoc environments. This paper presents an approach to the discovery of ad hoc services that exploits the fact that the relevance of such services is often limited to specific geographical scopes. Service providers define the areas (proximities) in which their services are available. Clients register interest in specific services and are subsequently informed whenever they enter a proximity within which these services are available. Since ad hoc services can be stationary or may be moving with the location of their mobile providers our approach supports discovery of services with fixed locations as well as of those that migrate with their providers. Our approach has been implemented as a push-based proximity discovery service and its evaluation demonstrates that it is well suited for highly dynamic networks as it maintains neither routes nor overlay network topologies.