With the popularity of portable computers and the improvements of wireless networking, there is a great deal of interest in developing applications for mobile computing systems. Causally ordered message delivery is required in several distributed applications particularly those that involve human interactions (such as teleconferencing and collaborative work). In this paper, we present an efficient protocol for causal ordering in mobile computing systems. This protocol requires minimal resources on mobile hosts and wireless links. Our message overhead in wired network is low. The proposed protocol is scalable and can easily handle host connections and disconnections. Our protocol, when compared to previous proposals, offers a low unnecessary delay, low message overhead and optimized handoff cost.