Surface ozone measurements at the four baseline U.S. geophysical monitoring for climatic change observatories provide a glimpse of tropospheric ozone behavior at locations generally isolated from local anthropogenic contamination. Variations on time scales from hours to years are considered. Observations in tropical and polar regions suggest weak coupling between tropospheric and stratospheric ozone in these regions. At subtropical and mid-latitude stations, on the other hand, there appears to be stronger coupling between tropospheric and stratospheric ozone. A small diurnal variation in surface ozone at Samoa may indicate photochemical influence on the ozone budget there. Some of the year-to-year differences in surface ozone at the four stations indicate possible global influences on the tropospheric ozone budget. No significant long-term trends are apparent in the data, however.