As portable computing devices become more popular, increasing the wide use of group communication applications that need reliable delivery of data, the need for a reliable multicast protocol in networks with mobile hosts also increases. The highest protocol for reliable multicast in networks with mobile hosts uses an ack-based approach and consists of mobile support stations (MSSs) and supervisor hosts (SHs). The MSSs communicate with the mobile hosts within their cells via wireless links and forward ack messages from mobile hosts to SHs. Each SH controls a group of neighboring MSSs and delivers one ack message to the sender, after collecting ack messages from MSSs, each of which contains some mobile hosts joined in a multicast group. In this protocol, if the mobile hosts within the domain of particular SHs are continuously crowded according to the regional property such as downtowns and suburbs, an ack implosion of these SHs can occur due to a number of ack messages from mobile hosts. Therefore, the multicast group communication is limited by the imbalanced processing overhead among SHs. To solve the above problem, we propose an efficient reliable multicast protocol in networks with mobile hosts.