Research in the concurrency control of real-time data access over mobile networks is receiving growing attention. With possibly lengthy transmission delay and frequent disconnection, traditional concurrency control mechanisms may become very costly and time-consuming in mobile distributed real-time database systems (MDRTDBS). Due to limited bandwidth and unpredictable behavior of mobile networks, the past research on concurrency control for distributed real-time database systems (DRTDBS) can not be directly applied to MDRTDBS. In this paper, we propose a distributed real-time locking protocol, called SDHP-2PL, based on the High Priority Two Phase Locking (HP-2PL) scheme and the concept of similarity for MDRTDBS. We consider the characteristics of mobile networks and adopt the concept of similarity to reduce the possibility of lock conflicts. A detailed model of a MDRTDBS has been developed, and a series of simulation experiments have been conducted to evaluate the capability of SDHP-2PL and the effectiveness of using similarity for MDRTDBS. The simulation results have confirmed our belief that the use of similarity as the correctness criterion for concurrency control in MDRTDBS can significantly improve the system performance.