The IEEE 802.15.4 standard allows devices to access the medium not only in contention mode but also in a contention-free way, in order to support quality of service (QoS). In contention-free mode, devices access the medium according to the guaranteed time slot (GTS) mechanism, which is vulnerable to the selective jamming attack. This is a particularly insidious form of denial of service that allows an attacker to thwart QoS while limiting her own exposure at the minimum. In this paper, we present selective jamming resistant GTS , a solution against the GTS-based selective jamming. We also show that our solution is standard compliant and affordable for resource-scarce devices like Tmote Sky motes.