Surface ozone concentrations have been measured continuously for the first time, in a forest preserve area near Manaus (3 S. 60 W) during the ABLE 2A field expedition of July--August 1985. Major subjects addressed are diurnal variations and processes that might control it, both dynamic and chemical. Intense transient variations have been observed in association with showers and thunderstorms. The diurnal variations are well defined, with minimum concentrations during the night and maximum concentrations in the daytime. At night it is not uncommon to find zero O3 mixing ratios, whereas the daytime maximum of between 7 and 31 parts per billion by volume occurs around 1200--1300 LT. Below a height of 6 m a very strong O3 concentration gradient is found. On two occasions, nearby thunderstorms have caused strong abnormal enhancements of the O3 concentrations. ¿ American Geophysical Union 1988