By using data from nine magnetometer observatories at widely separated locations about the earth the power spectral density of geomagnetic field fluctuations from 1965 through 1972 was studied for variations of periods ranging between 0.3 and 300 s. The north-south and east-west field components were about equivalent in size, averaged over a month, at all locations except the equator, where the north-south component prevailed. The vertical component was less than 0.6 of the north-south one eveywhere and varied greatly between sites. The activity was greatest at the auroral zone in the postmidnight to early morning hours; at middle latitudes the activity was greatest in the early afternoon hours. The equinoctial and winter months showed more activity at high latitudes; there was a summer month maximum at lower lattitudes. The slope of the log power spectral density versus log period curve was about 3 at low latitudes and 4 at the auroral zone. There is a maximum of power spectral density at auroral latitudes and a minimum at the lower middle latitudes. For the 10-s period the power spectral density varied from about 2¿10-5 to 2¿10-2 &ggr;2 s and for the 100-s period from about 4¿10-1 to 2¿102 &ggr;2 s. The correlation between spectral amplitudes for the 12-s period and sunspot number was close to 0.9 at College, Alasma. Earth surface conductivity, ionospheric variations, and magnetospheric conditions controlled the amplitude of the signals.