Energy saving is the most critical problem in sensor networks and dominates the design objectives. Due to the unique characteristics of sensor networks, to design energy-efficient modulation schemes, both the energy transmitted through radio channels, and the energy spent in the circuitry and MAC protocols, should be considered. In light of this observation, we propose a noncoherent FSK based flexible modulation scheme in this paper. We employ adaptive modulation and multiple simultaneous access which can be implemented by very simple low-power circuitry. A simple symbol synchronization algorithm is also given for the implementation of the proposed system. Simulation results are presented to illustrate the performance of the proposed modulation scheme. The approach employed in this paper can easily be adapted to different channel environments and application requirements.