Mobile agents often travel in a hostile environment where their security and privacy could be compromised by any party including remote hosts in which agents visit and get services. It was proposed in the literature that the host visited by an agent should jointly sign a service agreement with the agent's home, where a proxy-signing model was deployed and every host in the agent system can sign. We observe that this actually poses a serious problem in that a host that should be excluded from an underlying agent network could also send a signed service agreement. In order to solve this problem, we propose a secure mobile agent scheme achieving host authentication with designated hosts, where only selected hosts can be included in the agent network. We also present a security model and provide a rigorous security proof to our scheme.