The error in using transmissibility at the undamped natural frequency in place of the maximum transmissibility of a single‐degree‐of‐freedom system is less than 4% for all values of the damping ratio. The error is a maximum for a damping ratio of about 0.6 and decreases with both increasing and decreasing values of the damping ratio. The error in using transmissibility at the undamped natural frequency is less than the error in using theQof the system for all values of the damping ratio. [Research supported by the Electronic Components Laboratory, Wright Air Development Center, under Contract No. AF 33(616)‐5752.]