This paper studies the behavior of semiactive suspensions subjected to a pure-tone input. A base-excited system, representing a single suspension, is used to compare the vibration isolation due to a conventional passive damper, an optimal passive damper, and two different types of semiactive dampers. The semiactive dampers considered are on-off (bang-bang) and continuously-variable semiactive dampers. Similar to the optimal suspension, both on-off and continuously-variable semiactive suspensions exhibited the ability to lower the resonance peak without worsening the isolation at higher frequencies. The results further show that semiactive dampers can be useful for a variety of applications including sensitive machinery that cannot tolerate any overshoot in power-up or power-down, and yet must have good isolation during normal operation.