The concentration of atmospheric carbon monoxide, CO, was measured in the Amazonian rain forest. Air samples were collected at least three times a day during a continuous sequence of 36 days, from a height of 1.5 m above the ground. These surfaces CO measurements were part of a large binational field expedition to the Brazilian Amazon region, the ABLE 2B mission, a joint American-Brazilian effort to measure local concentrations of several species relevant to atmospheric chemistry. The time period of this experiment was April--May of 1987, in the local wet season. The measurement technique used was chromatographic separation followed by mercury vapor detection. The surface CO measurements obtained represent a comprehensive CO data set, unique in that the concentrations were measured immediately after collection, by a measurement system installed next to the sampling sites. The CO lab was set up at Ducke Forest Reserve, some 30 km north of the city of Manaus (3¿S, 60¿W). The results show a small but consistant diurnal variation of the CO concentration, with a maximum around noon. The average CO mixing ratio for the period was 102.9¿11.4 parts per billion by volume (ppbv), with most samples giving concentrations between 95 and 105 ppbv. On six occasions in April, large CO concentration were observed during relatively short periods, and these were classified as pollution episodes associated wtih unusual wind conditions which probably brought air into the forest that had contact with the city of Manaus. No such episodes were seen during May. Simultaneous sampling inside the forest, under the canopy, and in the Ducke Forest clearing, show larger concentrations in the forest, by a few ppbv. This result is consistent with the direct measurement of the CO profile below the forest canopy, which gave a concentration gradient consistent with an upward flux of 4¿1011 CO molecules cm-2 s-1. ¿American Geophysical Union 1990