We define a model of membrane system where each membrane is clocked independently from the others, in the sense that every derivation step is applied without a global synchronization. The computation is obtained by the execution of a limited amount of rules in each membrane, and only when they are allowed to execute a derivation step. Indeed, each membrane operates with a certain work frequency that can change across the system. Simple results show that this model is at least as powerful as the usual one, and the goal is to present a few examples that show it giving rise to interesting dynamic behaviors.