The energy loss due to the excitation of parasitic modes in the SPEAR II rf cavities and vacuum chamber components has been measured by observing the shift in synchronous phase angle as a function of circulating beam current and accelerating cavity voltage. The resulting parasitic mode loss resistance is 5 MΩ at a bunch length of 6.5 cm. The loss resistance varies with bunch length σz approximately as exp(-0.3 σz). If the measured result is compared with reasonable theoretical predictions, we infer that the major portion of the parasitic loss takes place in ring vacuum components rather than in the rf cavities.