Power spectrum analysis has been applied to the total ozone time series at each of a sample of Dobson stations with records of more than 15 years in the interval 1957 to 1981. The distributions of strongly periodic or quasi-periodic signals in these spectral estimates, for three spectral estimators and two levels of confidence, show distinct features. These appear at the previously known or suspected periods of 3.5--4.0 years, 27 months, and 21 months as well as the previously unreported period of 11.5 months. Possible geophysical implications are discussed.