Wireless sensor and actor networks are potentially one of the most important technologies of this century. There are many fields where they can be used in order to develop varied and interesting applications: high security environments, environmental monitoring, objects and events detection, military surveillance and precision agriculture. On the other hand, the ease of programming is a major barrier to the adoption of this kind of system. Recently, different high-level programming abstractions and middleware have appeared as promising solutions. In this paper, a middleware is proposed for USEME, a service-oriented framework focussed on the deployment of lightweight services in sensors and actors. The high-level model supported abstracts application programmers from implementation tasks such as discovery, communication, group formation or real-time constraints.