In　this　paper,　we　present　an　energy-efficient　dynamic　key　management　scheme　in　which　new　sensor　nodes　can　join　a　sensor　network　securely　and　compromised　nodes　can　be　isolated　from　the　network　in　time.　Unlike　in　centralized　and　location-based　key　management　schemes,　our　scheme　doesn＇t　depend　on　such　infrastructure　as　base　stations　and　robots,　thus　it　possesses　a　high　level　of　flexibility.　By　using　a　pseudo-random　function　and　the　elliptic　curve　digital　signature　algorithm　in　our　scheme,　energy　consumption　can　be　reduced　significantly　in　key　establishment　and　maintenance　phases.　Analysis　shows　that　our　scheme　has　a　very　low　overhead　in　terms　of　computation,　communication,　and　storage.